OK, I got a little more info on this regen stuff. It'S all messed up, so I will probably do some changes to it in the source
Right now, sphere.ini states that:
// Time in seconds for hitpoint regeneration
Regen0=40
Well that's not really in seconds, it's in tenths of seconds so 40 here is 4 seconds.
Now, for TAG.OVERRIDE.REGEN_0 in this case IT IS in SECONDS but it's not a total override like most people thought it was, it's a modifier to the default ini setting.
So if TAG.OVERRIDE.REGEN_0 = 3 in this case, you take the ini setting of 4 seconds, minus the 3 seconds of the override and your rate is down to 1 sec.
There are also other modifiers to take into account though, like Dex will affect STR regen and food level will affect all regens.
As for TAG.OVERRIDE.REGENVAL_0, this is in fact an override and the value set is the amount of the stat that will regen when it's time to regen.
As for setting it to 100, well, it can't go faster then 1 second.
Is this more clear now?