Suggestion: change command so installation date is part of the summary?
A while ago I added UBR info in Windows mk_inventory.vbs, this feels like something similar, although different OS and what not.
Not really familiar with rpm, so googled a bit how to get install date / time.
Would assume following change should work, although completely untested:
Change \t%{SUMMARY}\t into \t%{INSTALLTIME:date}: %{SUMMARY}\t to have install date in front of the summary.
Perhaps make a backup before changing. (-:
Maybe ask for a change on https://ideas.checkmk.com/