There are special-needs car seats with leg/foot rests available, but they are usually down below.
The best option is to rear-face her as long as possible. 2 is a minimum, not a maximum, as you seem to know; many children can rear-face to between 4-6 depending on their size in seats now available. What seat is she in now? If it's not one of the longest-lasting ones rear-facing, maybe we can recommend a few for you to start saving for to rear-face longer. How big is she (torso and weight?) Those factors will determine how long she can rear-face.
Once she has outgrown rear-facing, there may be solutions other than special needs seats (which are expensive, and often you only can get insurance to pay for one, so you want to max out conventional seats first, especially for a child who will need harnessing for as long as possible for support reasons.) One idea is building up a support platform, like we do for kids with broken legs etc. that need support. Another similar idea (often used in the same situation) is to use an exercise ball that is not fully inflated, so it holds up the child's leg(s) and is inflated to the desired level of support. Another idea, a beanbag chair. Etc.
Basically, anything that can be used to support the legs and is lightweight enough that it's not going to harm any occupants if it hits them in a crash will be fine. However, as I said, rear-facing as long as possible is the first choice, so just keep going with that for now.