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This is, to some extent, what the open problem garden is for.
A few points:
(a) I don't think MO should be an open problems / research topic repository.
(b) Undergraduate background is variously defined, and nebulous to actually quantify. (Compare to, say, post-quals graduate background).
(c) In the two or three fields I am familiar with, most of the time when such a reduction is seen to be possible, the reduced problem becomes so easy and simply approachable that it ceases to be a question of professional research interest. I know that this is less so the case for combinatorics, discrete math, and some other fields, but in general I don't think the proposed tag will be one widely used or well-loved.
I think it'd be much better to actually start something like Open Problem Garden or the AIM Problem List but for undergraduates. However, I cannot predict whether such a site will thrive: sometimes we want to save the problems for our own students.
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