Multiple-sources dominator trees: compact representation and fast algorithm?
I think it's really hard to do that. First of all, for one source dominator tree, there are several dominator points for a vertex. If we want to use "Lengauer-Tarjan algorithm", we can't avoid using tree structure. But how can we save a tree with less than $o(n)$ space? It means that for $n$ source dominator tree, we have to use at least $o(n^2)$ space unless we create a new structure which can represent dominator's information more effective.

For a dag, I remember there is a another algorithm to find whether two vertices share the same dominator for one source. Maybe it will help, you may search uva12671 for more information. (Actually, I am thinking a developing problem about this recently.)

Finally, I am also fascinated by it. If anybody has a new idea, please contact me anytime.