I understand that you want an easy system as a guideline and it could work pretty well for archetypes and low-level characters. Most of the data in a d20-stat-block can be converted straight forward, the only problems I see are skills, feats (abilities), spells and the attack boni.
For skills I personally would prefer two tables. Table 1 contains the d20-skills and the corresponding brp-skills. Table 2 contains the total d20 skill bonus (including feats and boni) from 0-30 and the percantage in brp. That way you can look at your d20 stat block and have a result by looking in the tables without the need to calculate anything. Additionally you can ignore all feats that give you a bonus to skills.
For weapons you can use a similar table, so that you can ignore feats that give attack boni.
For the remaining feats it is a little more complicated - instead of endless descriptions of feat-conversions you can either make some examples for common feats like power attack or a general guideline that each feat should give the equivalent of a 5% skill bonus (or whatever seems appropriate).
The spell-systems are different in a lot of d20 systems (you could use the skill conversion for star wars d20 for example). I think it would be best to just provide a table with the d20-level and corresponding brp-data like the number of spells, the total percentage to distribute on the spells and the max-percentage per spell.
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