It was a long night last night, and I'm probably rambling here, but here are some 'top of the head' thoughts... in no particular order...
I've been down that road of thinking--about lenses I'd like to have on my "future" D700 or equivalent, while using a DX body in the meantime. It can really make you go crazy at times, because if you just get the best lenses you can use on a DX body, sometimes they will work on FX, sometimes not. Case in point: the 17-55 f/2.8. It's a great lens, and getting affordable because of all the 'switchers' to FX. I have a D300, and finally just bit the bullet and bought one (17-55f/2.8 for $710, exc. condition used) as my 'normal' zoom. I realized the D300 was most likely going to be with me for a long time, so it actually became a relief to just get what essentially is the DX version of the 24-70f/2.8 (25.5-82.5mm) and to just get back to enjoying taking photos and not compromising my desire for pro quality glass. Most of my longer glass will work on FX if I ever actually go that route, but I've learned to really like the extra reach DX offers for the 80-200 f/2.8 and 300mm f/4 lenses. For me as long as the D300/D300s and future DXXX series of pro-caliber bodies are still around, I don't see quite the pressing need for full-frame, at least for a while. But that's just me...
In your case, you're strongly thinking full-frame, and you say you can do it now. So, keeping what I said above in mind, if I were in your boat, I'd probably make the jump to FX now and get the glass you want...the 14-24 is one of those "one of a kind" lenses, and I've seen landscapes shots comparing it to other quality wide angles lenses, and when you go big... the difference is actually startling. The 14-24 will be one of those legendary lenses they always talk about. So if you want one, nothing else will really do. It will work just fine on DX, and 14mm is not that bad, it is like a 21-36mm lens on FX. That used to be my extreme wide angle zoom in 35mm days--20-35. That's generally wide enough to do a lot of creative work, seriously. So don't let the 14-24 "isn't good for DX" argument really sway you, because it's just damn good. The trade offs are as everyone says: its big, it's bulky, it's somewhat awkward, no filters, permanently affixed hood... but it is the best in it's class imagewise, period. If that's the deal, go for it. You can always sell it if you can't live with those issues, because it's a lens that will always have buyers.
What I think will happen with your D90 as backup--you won't even touch it again. You might intend to, but given a choice as to which camera to shoot with, you'll go with the one that you're drawn to. At least from my experiences, when I upgraded my D50 (which I really liked) to a D300, I used the D50 for the first couple of weeks, then it just sat in my bag. Then it moved to my desk drawer. Now, I really need to sell it before it becomes worth nothing at all...and mine are both DX. I think it would be even more likely when your backup is DX, and main camera is FX. So, I'd recommend selling both DX bodies and all your DX lenses and putting that money into glass for your D700. Funny thing about back-up camera bodies... they seldom get used, unless it's a working pro's second, and often in those cases, they're usually both the same bodies and both in action at the same time (tele and wide) for quick changes without mounting and remounting lenses all the time.
Anyway, it'll be interesting to see what you end up doing.