After having the DSEL and bindings, more flexibility will be added, but this requires more work. Something like DynamicAny in the POCO libraries would be useful for this goal. One can think even further (serialization of widgets, etc.) but that's still quite far away.
Ability to create/move widgets to another container (dialog, window) at runtime or ability to disconnect/reconnect signals at runtime is a must have. Generally speaking, the features specifications & the runtime API design is something that should come ages *before* considering using a DSEL way of expressing stuffs, no? Looks like you somewhat take the problem backward to me, because even if you agree on which DESL to take connecting this DESL to the runtime api will likely be problematic as everyone one this list will bikeshed about how the runtime api will look, what features the lib must have, etc... and by the time those specs & runtime API are written in stone the DESL will likely be meaningless :) My 0.02$ Philippe