mergeinfo just tells you what revisions have been merged into your branch, that's all. You can get the same info just by looking at the output of your merge command, it won't really tell you any more than that.
I'm not sure what else you want, the merge commend will tell you what revisions and files were merged in.
Addendum:
SVN 1.8 was released, I've just checked the release notes and there is this bit of info:
In Subversion 1.8, the -r option restricts the output of svn mergeinfo
to the specified revision range, which is useful when determining
whether or not a given revision, or range of revisions, is eligible
for merging, or whether it has already been merged. In the following
example, r682012 has not been merged from trunk yet, while r26112011
has already been merged:
$ svn mergeinfo --show-revs=eligible -r 682012 ^/trunk
r682012
$ svn mergeinfo --show-revs=merged -r 26112011 ^/trunk
r26112011