Sorry to hear about your situation and if it's any consolation you are definitely not alone in this.
I met with a very senior regional director from a major Spanish bank recently and I asked him this very question. He told me that the reality is that they wouldn't bother chasing any clients abroad for the simple reason that if they managed to, for example, take someone's house back home, what would they do with yet another property but in a foreign country?
He said it just wasn't worth their while doing it.
And here comes my disclaimer....
That's just one bank and what the director told me. Every bank is different and every case is different. Everyone has to make their own decision as to what to do in their own case.
If you want to go down the dacion en pago option then I assume that to show you can't afford the mortgage then you must stop paying it. I did a similar thing to force my bank to change mine to interest only, something they were reluctant to do previously.
We are all in negative equity these days as the market continues in decline. If you want peace of mind then force the bank to lower your payments to something you can comfortably manage, otherwise take the risk, walk away and hope that what the director told me is true. If you really can't afford it then you have no option other than handing the keys back and counting your losses.
Best of luck.