There are several ways to stand the 1/48 on it's own with FAST Packs.
The action pose is the obvious choice. Just manipulate the toy in a pose where the weight is distributed enough so the 1/48 will remain standing. The picture of the VF-1J (without FAST packs) on HobbyLink Japan shows one. Note that even though this is without FAST Packs, it works with them.
The second is to either lean the toy slighty forward while angling the legs backward. Like stretching your own ankles, the feet will be angled forward while the legs are angled backward.
The third way is actually reall easy and looks the best. Angle the upper body just slightly forward and have the legs straight and the z-axis of each knee angled out. This keeps the mecha almost completely in a standing pose, but is solid and keeps itself upright in battroid mode with FAST Packs. It almost looks as if the mech is in the standard pose, but the torso is slightly forward. A note, the hips (Ie tips of the intakes) must rest upon the side of the fighter nose canards. If they sit in front like usual, you won't get the angle you need and the mecha will be tilted too far back.