We can now request people from our holdings and all, so:
Will we see anytime soon the mountainhome request military aid in the form of soldiers or attack orders?
I could see conscription being pretty easy to implement as it can simply be the liaison telling you to "expel" this or that dwarf or some number of able bodied dwarves back to wherever, but that would probably be too annoying if you couldn't refuse.
Given that civs are currently unable to muster armies, but rather send whatever warm bodies they can round up in the site selected to perform an attack, the logical basis for such a request is currently missing, and doesn't seem to be even in the vicinity of the already horrendously bloated To-Do list before the Big Wait, I'd say there's no chance for it in the near term.
If a soldier request feature was implemented, I'd expect the soldiers' exploits to be visible in Legends Mode, i.e. that they participated in this attack and that one, kills (or getting killed), etc. Another important question is whether the feature would be desirable for players. If you just get a request to sacrifice citizens, I'd expect it to be an annoyance to most players (although it might be another opportunity beyond sending away to get rid of stressed ones). Sending mercs and monster slayers (if you've got any surviving ones), might sting less. However, if there was a reward attached, such as the survivors returning with experience and possibly some useful/interesting items (medals?), it might even things out. This is deep in the Suggestion territory, however.
A request to attack a site could be interesting, but apart from being outside the short term scope, it wouldn't be trivial to implement it in a reasonable way. For such an order/request to be reasonable, the target strength has to somehow be compared to the military might of the player fortress so an attack makes some kind of sense (yes, there are real world leaders without any sense...). This is something that might fit better with a starting scenario where the fortress is some kind of military facility.