Not sure if anyone still wants to know, but changing the below offsets to all zeroes will clear all pokemon from the rental passes (save file 1 only) and allow you to add storage pokemon to said rental passes.
Save File 1
1st Rental Pass
0x23AE8 - 0x23AFB
0x23B74 - 0x23B87
0x23C00 - 0x23C13
0x23C8C - 0x23C9F
0x23D18 - 0x23D2B
0x23DA4 - 0x23DB7
2nd
0x241D4 - 0x241E7
0x24260 - 0x24273
0x242EC - 0x242FF
0x24378 - 0x2438B
0x24404 - 0x24417
0x24490 - 0x244A3
3rd
0x248C0 - 0x248D3
0x2494C - 0x2495F
0x249D8 - 0x249EB
0x24A64 - 0x24A77
0x24AF0 - 0x24B03
0x24B7C - 0x24B8F
4th
0x24FAC - 0x24FBF
0x25038 - 0x2504B
0x250C4 - 0x250D7
0x25150 - 0x25163
0x251DC - 0x251EF
0x25268 - 0x2527B
5th
0x25698 - 0x256AB
0x25724 - 0x25737
0x257B0 - 0x257C3
0x2583C - 0x2584F
0x258C8 - 0x258DB
0x25954 - 0x25967
6th
0x25D84 - 0x25D97
0x25E10 - 0x25E23
0x25E9C - 0x25EAF
0x25F28 - 0x25F3B
0x25FB4 - 0x25FC7
0x26040 - 0x26053
If you want the rental pokemon to look legit, just use PKHeX and set level and met level to 50, origin game and met location to none, ball to pokeball, and OT to PKTOPIA.
Maybe I'll find the offsets for save files 2, 3 and 4....if I have time and someone really really needs it....
Have fun
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