It turns out that 5-player capability was built into the games and was simply disabled for some unknown reason. Changing 2 bytes in the game enables 5-player mode.

 

IPS patch for 95 (PAL): https://www.romhacking.net/hacks/9380/

Detailed discovery info: https://nhl95.fi/public/NHL95_5Player_Discovery.txt

Also https://nhl95.fi exists and is cool

How to manually patch the games (PAL/Europe editions... But it sounds like they're the same in the NTSC/USA ROMs):

NHL ‘95 (PAL, HiROM, 1MB, .swc with 512-byte copier header)

1. File Offset $371D9: 00 -> 40
2. File Offset $371EC: 00 -> 40

NHL ‘96 (Europe, HiROM, 1.5MB, .sfc)

1. File Offset $5759E: EA -> D0
2. File Offset $5759F: EA -> 03

NHL ‘97 (Europe, HiROM, 1.5MB, .sfc)

1. File Offset $57952: EA -> D0
2. File Offset $57953: EA -> 03

NTSC versions here. 

NHL 95 (USA)-5 Players.smc NHL 96 (USA)-5 Players.smc

NHL 97 (USA)-5 Players.smc

Edited January 30Jan 30 by kidswasted
added nhl 97

Thanks Buds!

Just remember to set Port 2 to Multitap after initially opening the ROM in RetroArch. Then you can Save Game Remap File so you don't have to set port 2 again when playing that ROM.