PeaceBang kicked up a little dust about the proliferation of endings to prayers. I thought I'd comment here instead.
I guess one dependable truth about worship is that some words and things (like parts of buildings) expand and others contract. It helps if you take a developmental embyologist's eye to the ways of worship. When I see a dias in a modern Unitarian church, I can identify the contraction of a chancel. The unprogrammed Quakers are really radical -- and the exception that proves the rule -- because it is hard to make these associations in their worship.
Sometimes an element contracts so much that it disappears, and the only evidence of it having been there is the context of other surviving elements. Clyde Grubbs notes that saying amen is appropriate at the end of the sermon if the last part of the sermon is a prayer. But without a prayer, the amen has lost its context and I would give it the boot.
Or the words get changed out (for whatever reason) but you can get an idea of what works and what doesn't by comparing the current use with an "ancestral text." (I suspect Humanist worship in younger congregations in particular could benefit from identifying the function of elements carried over from pre-Humanist liturgies. I think the tendency to reject has undone the fabric of how elements interrelate. Ditto "praise band" Evangelicalism.)
Also, religion detests a liturgical vacuum and sharply uneven forms. Inserts fill in the contracted or lost items like new pavers in a crumbling brick sidewalk. Consider chalice lightings and "candles of joy and concern" which add non-verbal elements to a very, very wordy ethos.
Let's go back to the "amen and analogs" situation, which is obviously a problem of expansion. Because of the pluralist sensibilities common to Unitarian Universalist congregations, there is a value to creating some variety in responses to prayer and prayer-like constructions. But without understanding what the reason for the amen in the first place, the additions are arbitrary. (Those which mean peace are essentially blessings and, if used, I think should be clustered with blessings. So too PB's waggish addition of gesundheit.)
And there is another problem, of contraction. Amen is the affirmation of the people. I think the only reason a person leading worship should say amen (at the end of one's prayer) is to prompt the congregation. For a minister to read a prayer and answer it alone with an amen is to cut the laity out of the prayer dynamic: an unfortunate but all too common piece of clericalism found in Protestant churches.
So my recommendation for the "amen and analogs" situation is to find some simple liturgical acclamation of assent -- be it amen or what-have-you -- and let the congregation use it (or not: I'm not the amen police) plainly. If you can, print the acclamation in the order of service so everyone is, er, on the same page.