blank attack indexes in the vanilla scene.bin include:
0100, 0101, 0102, 0165, 0166, 0167, 0168, 01B2,
0244, 0250, 0265, 0266, 0267, 026C, 0286, 02D1,
02D4, 02DC, 02DD, 0302, 0316, 0317, 0318, 031A,
031B, 0324, 0326, 0327, 0329, 032A, 0335, 0350,
037F, 0380, 0381, 0382, 0386, 03A0, 03A1, 03AD,
03C1, 03C2, >03E2
I don't know how high they're allowed. I assume they could go up to FFFE if you wanted them too.