Interesting idea. Here's a twist: what if the committer could set the value of their commit instead? Social pressure would dictate that most people would not overreach. Then those who make small but useful commits could recognize in advance their minor nature. This is essentially a kind of honour system.
That's actually a good idea; you'd probably get fairly reasonable results that way. The downside: using those tags at all makes it unambiguously clear that you care about the tip4commit tips.
One plausible possibility that might help: in the block that normally contains Signed-off-by and Reviewed-by, add a tag for marking a commit as "minor" or "trivial", in the sense used by the Linux kernel's trivial@ patches or the GNU project's threshold for changes accepted without copyright assignment.
And as a quick hack, scale the tip by log(diffstat) with a sensible upper bound.