ShapeShift started to use mixin and had to stop due to problems.
Coming soon there will be a service that allows BTC -> XMR transfers supporting mixins 3+ by default and payment IDs.
Would it be difficult to make, for example, buy ppl running xmr.to? Now xmr.to does xmr->btc, but the reverse service with strong take on privacy and anonymity, could also be useful I think.