According to my knowledge, following a vegetarian lifestyle to avoid eating non-halal food is recommended. But if you are trying to organize meat for the inmates then maybe you could ask a scholar to sit down with you and postulate an exception on why following a permanent vegetarian diet is not permissible for a Muslim in incarceration.
Vegetarian diets have to be put together carefully to avoid malnutrition in certain minerals and proteins. I am sure the prison will lack such an attention to detail and just pick the cheapest set of meals out of a cookbook. This is a strong motivator to organize meat, if circumstances permit.
Male inmates need to additionally build up muscle mass after a while in prison to undermine their presence as it is expected from them in prison culture. You can not do this with cheap carbs and plant-based proteins. Maybe that can go into a scholarly statement so that is tenable by fegh'h.
Also, if the fish is killed in a halal way, then the inmates can practice 'Pescetarianism'. Maybe one could convince them to serve fish as the Islamic meat option if it is easier for them to organize.
Edit: the common malnutritions are vitamin B12 and D, iron and sometimes calcium. About 90% of vegetarians develop this. Though it is easy to relieve this by taking supplements. If sports are concerned though, one will likely miss half of the needed protein varieties for a reasonable post-activity meal.