you can always use a very lightweight interfacing- which could be left in but the extra weight is next to nothing. of course you can piece without a foundation- it isn't as easy, but is certainly 'do-able'
the wash away products tend to be a bit expensive- but lightweight interfacing is pretty cheap. I've also used the least expensive, lightweight muslin i can find- the 99 cents a yard stuff...it too does not add much weight- interfacing is really easy to quilt through too- don't even really know it's there.