Hi rogue, yes it is possible. IMHO, it all depends on how you want to build your portal system. You got to decide if it would be feasible to do it with a dll written with C/Asm depending on your portal engine design.
Some things you may have to pass back and for to the dll is a bunch of polygon data that make up sections of the level. You may want the polygon data to see if your portals are visible are not down to pixel perfect level. Visible portal recursion techniques may take advantage of C/asm speeds.
There's alot of factors to consider, but it's definetly do-able with dedication and time to do it.
Some things you may have to pass back and for to the dll is a bunch of polygon data that make up sections of the level. You may want the polygon data to see if your portals are visible are not down to pixel perfect level. Visible portal recursion techniques may take advantage of C/asm speeds.
There's alot of factors to consider, but it's definetly do-able with dedication and time to do it.