I have seen posts (not sure where) where someone used an osymmetric ring without problems on an exogram crank. The exogram has the same 5 bolt pattern, so that should not matter. However, the bolt pattern is 'flipped' compared to conventional shimano/sram patterns. Therefore you should make sure you put the ring on 'flipped' as well (the pin should thus be placed opposite the crankarm instead of at the same side). Otherwise, oval chainring will only make you slower ;)

Using a round inner chainring is no problem. However I think you would be better off to use both oval chainrings, as you adopt better.