Dear Stigweard,I've been playing mandolin for at least 35 years in sessions and love to use chords occasionally, both when someone takes over the melody line within a tune, and sometimes when a tune is new to me. Wherever you read that playing chords is not acceptable is absolutely wrong.
Mandolin chords can make a very positive contribution to the overall sound of a session, provided that such backing is sympathetic to the melody and does not become monotonous.
Here is a site with mandolin chords to get you started but it is fairly easy to make up some nice "modal" sounding ones of your own!
Happy picking and strumming!
mooman