It could be like taxes, the alliance takes a certain percent of the nations military. Then the nation cant access it so that it forces alliances to be more cohesive and act quicker.
imo you shouldn't put a limit on the amount you can send or the time that you can send them so that we can have huge proxy wars start between two tiny nations with like 1mil troops (but still require the nations to foot most of the bill for these troops or make a limit where it costs more to keep them). This would also add an element to the early parts of wars where an alliance can stockpile troops to counter a blitz.
Then you could put a troop trading system, where large nations could sell their military on the market for a quick buck. Or alliances could support eachother in proxy wars through massive trades like the trade system we have now (like the rss trade but with soldiers, imagine the amount of meta gaming and stuff we could do with that).
(and allow alliances to send these troops to anyone willing to accept the trade for them)
And even if you've sold your troops on a lease you still have them taking up your capacity.
And instead of the troops being stored in an alliance bank sort of thing, they could be stored in a nation where the alliance chooses to send them (where that nation could not send them out if blockaded).
sorry if none of this makes any sense, I'm really really tired