What flavors work well within a splenda-sweetened strawberry-rhubarb cobbler?
The classic flavour pairing with rhubarb is vanilla: rhubarb and vanilla custard is a British staple. Fresh custard (Creme Anglaise, as the French call it) is easy to make and delicious. 

Almond also pairs well with rhubarb. You could incorporate ground almonds into the cobbler mix, or perhaps fold some amaretto liqueur into lightly whipped cream to serve with the cobbler.

As Splenda has no real flavour, just a sweet taste, both of these will work with it easily.