There's not really a huge amount to be honest Jamie. The problem with heads up is it's usually a case of biggest wallet wins. You need some leveller, like using the same motor as O/FF do, or a weight per cubic inch limit like Super Modified.
Super street, super gas and super comp are all index classes with 10.90, 9.90 and 8.90 dial ins. Probably closest to ET brackets, but the dial in time is given! You can still break out though, same as in bracket racing. As far as I know, no VW races in any of these classes.
Street Eliminator is heads up, but it has a 13.50 cut off, and cars must be in street trim (pump fuel, treads, etc) and the top cars are now running 7's at 180mph+. Martin runs in S/E with his '52.
Bracket classes are Sportsman ET (12.00 and slower), Pro ET (11.99 to 9.00) and Super Pro ET (8.99 and quicker), but they're still bracket, like VWDRC. Nigel Green is the only VW in Pro ET with his low 11 second 1303 (2165 with nitrous), no-one is in Super Pro, and SP/ET is entered by VWDRCers when they haven't got a round (like Euro Finals).
Comp Eliminator is a wierd one - there are dial in times depending on weight and engine size, but I don't understand the rules for that! Martin and Adele have both entered Comp Elim, alongside Russ Fellows.
O/FF is probably the best bet for heads up VW racing, but you are right in that it's not ideal if you want to race other stuff, unless you count the CHP Ashes and the general RWYB meets that we go to.
That any good to you?