Finn,
Being involved with last years Speedweek event and again for 2011 I can tell you there are definitive rules available for any competitors wanting to build a vehicle for the 2011 event.
In principle there are two overarching "rulebooks" , the first - 36HP Challenge rules as defined by Burly B
http://www.saltflats.com/36_HP.html
The second rulebook is from the actual organisation who run Speedweek here in Australia and for 2011 their rulebook is available at;
http://www.dlra.org.au/index.htm
Now when you put these together there are a few differences as you correctly point out with the primary one being that in the US you do not need to run the vehicle with a roll cage whereas in Australia the roll cage is a must and you will not be allowed to run. Given the extra weight carried by a rollcage ( up to 50 kgs) there needs to be some allowance and Burly B will usually authorise a reduction in weight in Australia by removing things like rear trim, door trim and rear seat squab. Oz rules also require steel valve stems and caps for each wheel whereas in the US this is only required if the vehicle top speed exceeds 100 mph.
Now a word of caution. In 2012 the DLRA have indicated they will be adopting the rulebook from the USFRA which is different again to the current DLRA rules so if anyone is building a 2012 vehicle I would recommend they liase with the DLRA to ensure they are working with the correct rulebook. I dont believe the 36HP Challenge Rules will be amaned but Burly B may be able to confirm.
Its all clear as mud,
Countrybuggybill