I've been playing with a similar problem. At my test site, I have one ADSL line, and I make use of 2 different ISP's, both with dynamic IP's. The one ISP is IS, and the other is SAIX. They both have different gateways.
The above is easy to do, but the problem is this:
Due to the fact that SAIX is good quality bandwidth, but capped at xGB, you need more than one account (unless your 3d tier ISP allows per GB billing).
When using ONE MT to dial both the IS account and SAIX account, you can route certain traffic over certain gateways, no problem. BUT, the problem comes in when you use more than one account from the SAIX network. These accounts always have the same gateway.
The only solution I got for this problem, was similar to XP2000's. For each account I dial out, I have a MT NAT/Masq the traffic. All these MT's plugs into the CORE router where all routing are done. This way routing is very simple and straight forward.
I would like to see the interface based routing to work, but I can see many NAT problems with it.
If someone manages to get interface based routing working in a similar case, is it possible to post the basic route planning and perhaps a part of your config?
The only thing Mikrotik lack, is to send power to your High Sites wirelessly.....