There are lots of creative ways to raise money - depending on what you want to do/which groups you ride with. Some MS Rides have other fundraising rides or runs during the year, and you can get credit for volunteering. You could have your buddies over for a chili potluck (they bring sides, you cook chili) and ask for a $5 per head donation. I've always been surprised that the people who really couldn't afford to give much gave in much higher percentages. TE folks have been very generous to me in the past. Lots of ways to get it done.

There are certainly charity rides that have no donation minimums (they usually don't really focus on fundraising other than the registration fee). I've not felt pressure at those to fund raise. Another option would be to find an organized ride near you that's not necessarily fundraising. Our local bike club does one - the proceeds from the registration fee are donated to charity, but they don't seek additional donations to my knowledge. If you don't really care about the charity and you're just doing it to have support, this is probably your best option IMO.