I think the method I used to arrive at a league adjustment is more scientific than the WHIP you pulled from your ass for sinkerballers, and yes, I think you can calc the adjustment. Don't give me the BS about the quality of the pitchers in the AL vs. the NL as some insurmountable barrier. There are good and bad pitchers in both leagues. The run differential is the difference in rules (the DH).
The new BBRef has league splits, but the splits don't have advanced batting metrics, so I'll approximate the run difference using the original RC formula (OBP * TB ). I'm using the 9th in the order split for the NL because the pitcher usually hits 9th and this will account for their PH replacements.
2008
[table]League|Position|OBP|TB|RC
AL|DH|.339|3535|1198
NL|9th|.244|2172|530[/table]
For the AL, 1198 / 14 (teams) is 86 runs per team, 86 / 162 is .53 runs per game for the position. For the NL, 530 / 16 (teams) is 33 runs per team, 33 / 162 is .20 runs per game. Keep in mind, the RC used doesn't account for SB or SH, which are both positive contributors in the contemporary RC formula. The AL DH rarely steals or sacrafices himself, whereas the NL pitcher regularly sacrafices and the pinch runner is regularly used (the difference in SH+SF+SB for these spots is 706 for the NL vs. 143 for the AL - in the contemporary RC this category has a .52 coefficient). Additionally, this approximation does not account for subsequent pinch hitters for relief pitchers at other batting spots.
The difference using this quick version analysis is .33 R/G, with the actuals being .24, .19, .21 the last three years respectively. If the analysis were more in depth and accounted for SB, SH, SF, and the other PH's used for relief pitchers, I have no doubt it would be right in the .20 range (only 300 total addtional RC is required for this, and you almost get that with the quantity difference * the .52 coefficient for the SB+SF+SH category).
When the effect of the rule change accounts for the difference in scoring, you can do one of two things. One, you can assume that the rule change is the difference and all other things are equal. In layman's terms, this means the quality of the talent in the NL and AL are the same. This would be the Occam's Razor choice. Two, you assume that the rule difference is not the only difference and that all other differences in talent, parks, weather, etc are in a harmonious balance that makes it look like the only real difference is the rules, and that this balance occurs every year. Color me skeptical.
So, back to the original discussion. Yes, I think a league adjustment is perfectly reasonable, and I'm comfortable saying Brad Penny would have been a 4.00 ERA pitcher in the 2007 ALE. No amount of unsupported league difference rhetoric is going to sway me from that. You'll have to prove it.