not sure how your congregation works as i've not attended a catholic church, but in our messianic congregation sometimes there is light piano music as prayers are being said. at first i was surprised that this was allowed, but it was actually very beautiful. we have a special situation because the spiritual leader's wife is a pianist and conductor. also, music is played occasionally before the service and obviously the piano is used when the choir sings (not always to the organ). usually with the formal service the organ is used, but to maintain eye contact the piano is used with the choir. the reason for this is that the organ is way up in the loft and the choir members would either have to sing from there (which means getting up, walking up the stairs, and taking a while to get there). getting up from their seats and singing within 1-2 minutes is much easier and allows them to see the the music leader from the piano. since i joined the congregation - my husband sang, too - so he likes the comfort as a soloist of being in the L of the grand piano (sort of as a back drop) instead of completely alone on stage.
i am totally with you on the piano being a beautiful addition to the music service. jazzing up songs that are sung - is another option. you can play lightly - using several hymns put together - very lightly as people are entering or exiting the service. it is a very nice touch, imo, and lets people know 5 minutes before they should sit down. otherwise, people are milling around and don't really watch the time. (unless the organist does this already).