Hi!
Bent resting poses are quite common for rigs in toe-walking creatures. Just use a bit more advance fastrig (m-gear, Advanced Skeleton, Rapid Rig) that lets you pose your proxies before you create the rig, and you shouldn't have a problem.
If you want your leg in default pose to be straight, I would do the following:
1) Place your bones in the bent leg
2) Rotate them into a straight line
3) snap a new joint chain to that straight line
4) rig the straight joint chain as you normally would, this way you can avoid a lot of the issues that come with bent legs and use your normal workflow.
5) Before skinning rotate your joints into your leg
6) Skin in the bent position
7) When you reset to bind pose the leg will straighten out
I hope it helps!
