On the html: well, I'm not a themer, so I may be wrong, though with just a "simple" markup like:
<li class="listlevel1 icons">
<span class="membergroup_1" />
</li>
you can attach css to:
.icons:before
.icons
.membergroup_1:before
.membergroup_1
.membergroup_1:after
.icons:after
it should be a fair amount of places to customize the badges, no?
emanuele is thinking at some setting to expose font-awesome icons...
Actually, a way to let people use FA icons where possible/meaningful would be a nice thing, I think.