Yes, I describe in the Tips & tweaks thread how to get 720p 50Hz (in Lx3 obviously). You need to reduce V.Active to 240 and increase V.Backporch as needed. Ofc, ymmv as the display needs to be able to handle having the excess lines in the backporch.
As I don’t have an OSSC currently, I haven’t tested this with recent firmware, where V.Frontporch can also be adjusted. Depending on display, I imagine it could be better to divide the porch increase more evenly into both front- and backporch.
I doubt you can achieve correct AR in Lx2 though, since 480p 50Hz is not standard.