modern alternative to collagen for a flexible gesso?

There are a proprietary silverpoint grounds made by art materials manufactures available. These appear to be bound in an acrylic dispersion or PVA dispersion. A quick Google search will give you some choices.
As to historical grounds on paper. The papers are generally sized with glue making it much stiffer. The “ground” is very thin and not like one used on a panel for egg tempera. I have my students use animal glue with ground silica rather than chalk. This is tinted with dry pigments. I have not seen any cracking on their works unless they deviated from instructions and piled the “ground” on the substrate.

Great questions, lots to say in response, I’ll try to be concise.  

A good metalpoint ground is abrasive to a metal nib. To create this “Toothy Hardness” a ground needs:
a.  High Pigment Volume Concentrate (PVC); i.e. the ground has a high pigment (solid) content relative to binder, so pigment particles protrude above the surface of the binder (to create a microscopically irregular and thus abrasive surface)

b. Hard Pigments; i.e. pigments (solids) that have a high “Mohs” or MHS rating.  

Also helpful are pigments that have a rough, irregular shape (such as historic and natural earth colors) and moderate particle size (neither too big nor too small; i.e. .1 to 2 microns) – however these latter two characteristics are not as important as high PVC and pigment hardness (MHS rating). 

Although not essential, it’s also helpful if a metalpoint ground is water-based = non-toxic, dries quickly, easy to clean up. Other ground considerations include: cure time, solubility, number of layers, sand-ability, opacity, value, flexibility, prep time, natural or synthetic, durability. 

Given the above, there are many options for metalpoint grounds; and, consequently, many different resulting characteristics.  Grounds with a flexible binder, not too high PVC, applied very thinly and/or in just one or two layers (i.e. a thin layer of white gouache) can tolerate a flexible support such as paper.  Grounds made from less flexible binders, with extra high PVCs, applied in thick or multiple layers are more apt to crack and thus need a rigid support.  

Harder, more resistant supports (wood or metal based panels) are another way to increase abrasion; conversely lots of “give” in a support (paper) can lessen abrasion – but this consideration isn’t as important as the ground itself. 

It’s true that there are old metalpoints on vellum, but many are on prepared paper.  In the latter you often see how thinly the ground is applied – a light wash of gum Arabic or animal glue combined with a bit of bone ash, for example – because the characteristics of the paper show through the ground. 

When I apply a ground to paper – even if it’s a thin, flexible ground – I first attach the paper to a solid support, such as an ACM or MDF panel.  It gives more long term protection to the drawing and I like the resistance of the underlying solid support.  But, as long as your ground has some flexibility and/or is very thin, it’s not necessary to mount paper to a solid support.  To test the flexibility of a ground, apply whatever number of layers you plan to use to something like 90# watercolor paper, then gently bend it in various directions and see how it reacts. 

As for abrasive pigments to use in a ground, here are some options (followed by their approximate MHS)
 Glass Powder 5.5 
Chalk 3 
Bone Ash 3 
Barite 3.3 
Zinc Oxide 4 
Feldspar 6 
Pumice 6 
Pyrite 6.5 
Titanium Dioxide 6.5 
Quartz 7
Silica 7

The dust of any pigment is a lung irritant, but particularly beware silica, a known carcinogen.  Wear a mask, not only when adding pigment to a ground but when sanding the ground.  Different fillers each can create unique results: for example, I found pumice beautifully enhanced the individual color in metal marks; silica, glass powder, and bone ash did as well, to a lesser degree.  I now always add a percentage of one of those to my metalpoint grounds.   

There are SO many variables in a metalpoint drawing – many different potential supports, grounds, additives to grounds to increase abrasion, metal nibs, environmental factors – some of which are unpredictable and not even quantifiable – that that there will always be inherent variability to the production and life of a metalpoint drawing.  Most metalpoint artist I know see this unpredictability as one of metalpoint’s distinct charms. 

What fun – enjoy!

Koo

Koo, so glad to have you as an official moderator.
I totally forgot to mention what you stated. I always have students stretch and then size the paper, then allow it to dry before they apply the very thin ground. This maintains the planarity and also provides the rigidity necessary for working. You can then either do the metal point drawing after the ground has dried but paper remains adhered to a rigid surface (wooden drawing board in our case) or remove the sized and grounded paper and proceeding with the application of metalpoint.

Important point that most papers (excepting, perhaps, extra heavy weights such as 300 or more) should be stretched first – glad you mentioned that, Brian.