Multiple instances of Reflect stack additively
From what I can tell, OpenMW stacks multiple instances of Reflect additively, resulting in a character with a total combined Reflect of 100% or above being totally immune to all non-reflected magic attacks. This is in line with the description on https://en.uesp.net/wiki/Morrowind:Reflect, which is different from what I've found.
Here's a video from a basic Morrowind installation, no mods, no MCP (with MGE XE):
Equipment:
Wraithguard (20%)
Marara's Ring (20%)
Darksun Shield (20%)
Robe of Drake's Pride (25%)
Helm of Graff the White (10%)
Dagger of Symmachus (10%)
Total: 105% combined Reflect
Spells get through on a regular basis. I've tested for 100 casts out of which 35 were not reflected. If multiple instances of Reflect stacked multiplicatively like they do with Absorb, the equipment above should provide an average chance of 69% to reflect (if my calculation isn't wrong), which is not too far from my small sample size.
According to the comments on the wiki's talk page I'm not the only user for which Reflect works this way, so if others can confirm this behaviour I'll assume the entry on the wiki regarding how Reflect stacks is incorrect.