Ah, sorry, I maybe didn't explain things very well. I'm not intending to use PoW universally, as anti-spam in every transaction; although that's certainly a future possibility, for both Ripple and Splash to consider, if the existing regime of charging a small fee that escalates on signs of "spamminess" proves insufficient. For now, the existing charging regime seems to be working fine, in Ripple, and I intend to simply use it unaltered in Splash.

My usage of PoW will be technically far more humble (and yet, philosophically, quite pivotal): namely, as the unique way the native asset (XSP) is ever born. Once born, though, it then behaves just like the XRP in Ripple: it moves around via ordinary (non-PoW) transactions, acts as a path shortener in order book matching etc, and is the unique unit for denominating reserve requirements and anti-spam fees. And none of that activity will involve PoW at all.

About possible mixed systems: yes, it would certainly be possible to have a Ripple fork with both an "XRP" (either owned entirely by the fork's developers at launch, or snapshotted from Ripple at the fork moment) and an "XSP" (born gradually by PoW). I'm not sure that path-shortening or anti-spam would really be helped much by having two native assets rather than one; but I suppose if some people philosophically prefer the one kind of native asset and other people the other, they can express their preferences in their holdings while still all being members of the one giant mixed system. (And thus able to trade with each other.) My own development preference is to build a "clean slate" self-contained system. But other people can fork Ripple (or indeed Splash!) any way they please. I hope, if they do, they choose to document their development adventures in this forum, or some other prominent place on the web.

(I'm intrigued by your mention of BTC or similar as a possible native asset. I presume you don't mean an IOU that happens to be spelt with the letters B,T,C? Do you mean some sort of crossover protocol, where burning of blockchain BTC causes something called "[non-IOU] BTC" to appear in Ripple shortly afterwards? [There'd be no crossing back of course. Not unless the Bitcoin developers, ah, got the Ripple bug. ] Or do you mean something else entirely? Apologies for sometimes being slow to understand things. - Again, whatever your exact meaning, this would be for someone else to try: I'm going to try a clean slate.)

And now, finally, as I've said in my edit of my "(reserved)" post up at the top, I'm afraid I'll be away from the net for a fortnight. So don't be alarmed at my falling silent after this message. Rest assured, I definitely plan to conduct my development process in the open... probably on GitHub I'd imagine. - I should confess right away, here and now, that I'm still a novice at the management of a really large, heterogeneous body of code. Put it this way: I still haven't got Ripple to compile yet on the machine I'm using, so I'm nowhere near ready to start tweaking it! But even if Ripple forking defeats me utterly, I'll still aim to put pseudo-code on my wiki page, with the hope that someone else has a go at turning it into a real, working fork.

Thank you for wishing me luck - I'll need it! See you all in a fortnight.