You know, considering the setting and conditions of the performance, maybe it's best not to play at all? I love playing bass, but, back I was playing for my church's praise team, I sometimes found myself thinking, "Is a bass really necessary here?" This often happened in small, quiet settings. I thought the pressence of bass frequencies (especially those as strong and growly from MM basses!) would disturb the setting more than help it.
If you really don't want to use your guitar amp, then have the choir director help you lug the Cerwin Vega up the stairs. After all, it was he/she who asked you to play in the first place, right? Only seems fair.