Why don't we just copy the IRB rankings? Make it so every competitive game played domestically or in Europe scores/loses a team points based on the number of points each team currently has. Form the tiers out of the rankings. Still not totally accurate, but may be the best we could do.
This actually does exist, and wouldn't differ too drastically from what the ECC rankings are:
http://www.eurorugby.com/index.php
For example, their tiers would be:
Tier 1: Toulon, Leinster, Saints, Saracens, Glasgow
Tier 2: Munster, Bath, Ulster, Leicester, Quins
Tier 3: Clermont, Racing, Castres, Montpellier, Toulouse
Tier 4: Sale, Scarlets, Ospreys, Wasps, Treviso
Compare this to the ECC tiers:
Tier 1: Saracens, Leinster, Toulon, Glasgow, Saints
Tier 2: Munster, Castres, Leicester, Montpellier, Racing
Tier 3: Clermont, Quins, Ulster, Bath, Ospreys
Tier 4: Sale, Wasps, Scarlets, Toulouse, Treviso
So tier one is identical, while tier four differs only by one side (Toulouse instead of Ospreys.) Obviously tiers two and three are more mixed, but this is due to the semi-random nature of the ECC draw, with teams dropping between pools essentially based on the order that they're drawn on. However, given that most of the sides in these two tiers are of comparable strength (Clermont being a possible outlier) this shouldn't have a large effect, given that the upper and lower boundaries (ie. tiers one and four) are essentially fixed.