1) Send bitcoin to address through blockchain.info with a note attached containing your monero address. (or alternatively digitally sign your monero address with what ever bitcoin address you send from)
2) I use that much bitcoin to perform a market order on the exchange
3) I send how ever much monero I am able to buy to your monero address
Would anyone be interested in this service. You benefit because you don't have to make an account a poloniex or find people in the otc. I benefit because people buying pushes up the price and i hold a monero or two myself.
The only thing is that maybe going onto the exchange and buying yourself is easy enough that this isnt worth it but im curious what you guys think. if this sounds like a service that would be super valuable to you than who am I to judge.
Would probably be more interesting if you make it as automatic as possible (ie, immediate) with a fixed price. Fixed price not being absolute of course, but something like poloniex + N%.
Not knowing how much XMR you'll get when sending your BTC would be very hard to most people, I think.