I've never made b'stilla (though I might try soon), but after some experience with middle-eastern rice dishes I have the following advice:
- Getting nutrition, flavour and consistency are all important.
- Chickpeas are very good nutritionally, and also fit in with the regional flavours.
- It's improtant to get the sweetness right. I usually use onions and carrots, finely chopped and sauteed for a long time, to which I add the cooked chickpeas.
- The long cooking sorts out the consistency as well.
- You can either give up on the full-meat flavour, or try to get close to it using Worcestershire-like flavours. A tiny bit of tamarind paste and some sweet soy sauce will go a long way towards this.
I've tried Seitan, and the consistency is wrong.
I've put some shredded soy stuff (meant to simulate ground meat) to good use in this sort of thing, but you need to flavour it right as it is as bland as something very bland.
It would work well to use re-hydrated TVP. I have had vegan shepherd's pie and it maintains the kind of consistency through the cooking process that you are going to want in the pastilla / b'stilla meat(less) pie. Browning the TVP could prove challenging but if you make some kind of ground combination including a wild-ish mushroom (like cremini) and beets it could go aways to allowing it to brown to a very similar texture as the meat in pastilla.