This thread came up a lot when I looked around about this, I intended to post this several months ago. Unfortunately I wanted to upload the mod to mod sites and do some editing, but I haven't gotten around to do it yet.
Edit: Now I have! It is available on Nexus Mods.
Anyway, I have created a little mod which fixes these birthsigns, here is a short description:
Vanilla Birthsigns for OpenMW by Klorax changes the behavior of the attribute enhancing birthsigns (the Lady, the Lover, and the Steed) to match that of original Morrowind.
This is accomplished by replacing the constant magic effect with base attributes upon finishing character generation (i.e., when leaving the census office building). When this happens the player is notified. Thus you need to start a new game to use this addon.
In original Morrowind these constant magic effects for attributes are treated as base attributes. This is inconsistent to any other constant magic effect (i.e., spell or enchant) for attributes. OpenMW removes this inconsistency, which is good IMHO, but this makes the original birthsigns behave differently (mostly less powerful), and this addon aims to fix this.
This means that the provided endurance will now count for level up health. All relevant attributes (agility, endurance, personality, and speed) will now count towards skill level caps for governed skills. The attribute base values (through leveling) will not be able to get past 100.