Just wear some good gear. If you are mountain biking anyway...you should probably have some gear on.
A good lightweight armored textile jacket will keep you from getting road/tree/rock rash and it will protect you MUCH better than nothing. The air goes right through them too, so you stay cool (but it will definitely be hotter than a tshirt).
I have the same concern on my motorcycle. You can bet that I wear gear head to toe...no way am I damaging my art, that cost a lot of time, pain, and money.
Here is the kind of jacket I'm talking about
:
http://www.kneedraggers.com/details/...521-FIR-8.html
If you pull out the insulated liner...its like super tough fishnet material...very breathable but still has armor for your arms, shoulders, and back.
Now, this was designed for motorcycling...but you might find something similar for mountain biking...or just get a jacket like this and ride in cooler weather.