Yes, you can use CustomActions to act as FF/REW. Hide the queue (leftmost button) by setting an empty queue on MediaSession and add exactly two custom actions (your custom REW and FF) to the playbackstate. The first, REW, will be placed on the leftmost slot, where the Queue button usually sits (emptied by setting an empty queue). The second custom action, FF, will be placed on the rightmost slot.
If you set more than two custom actions, it will activate the overflow menu and the rightmost action, which should be FF, will be replaced by the "more" (...) button, moving the FF custom action to the overflow menu.
Also, make sure you set the extras for ALWAYS_RESERVE_SPACE_FOR.ACTION_SKIP_TO_NEXT and ALWAYS_RESERVE_SPACE_FOR.ACTION_SKIP_TO_PREV, so if you eventually disable NEXT or PREV, your custom actions don't move around.