Yea thats interesting.... I thing the main challenge will to separate Splinterlands assets, since those are like almost native to the system, for example there might be DEC valued in 10M.
If you remove Splinterlands, then you can just go for BTC,ETH, LTC, and few others, and HIVE and you will have some rough number.
I think that native assets should be included, as they're part of the value that is circulating on Hive-Engine. What should be excluded is all the weird low float low volume tokens that don't have enough interest for meaningful price discovery.