There are rod brackets that you can mount ceiling.

Lots of examples here,

curtains ceiling rod bracket

I don't know for sure, just guessing, but probably get L shape support brackets and mount them to the ceiling. Have them turned so that the side that drops down comes where a wall would be and then screw your panel onto it. It looks like there are ceiling mounted curtain rods, but they just have a ring so that won't work.

my problem is there is no ceiling where the center mount would go- the bay window ceiling is behind-

All gave you good advice..I am sure you will come up with something to keep the long valance from sagging.....Just have to ask, since you have shades will you be drawing the drapes? If not, just put two small rods on the wall (ceiling height) and put your drapes on that....no valance. Is this a bedroom or a child's bedroom? I don't think I would want a heavy looking valance on that bay window.