It would be difficult to produce a wireless version of the I-PAC. The main problem is all other considerations (cost, performance etc) have to be put aside to focus on battery consumption. This means a complete redesign using low-power and possibly low-performance chips.
The cost is much higher as the device is split into two halves, the dongle, which ideally would need to be in a housing, and the remote side. Then there is the physical issue of providing some kind of battery housing on the remote end.
I am not clear where the demand lies either? By their nature, arcade controls tend to either be in cabinets or in a large controller which is located near to the PC. I am not sure if having to mess around with battery replacement and possible performance issues etc is worth the small advantage, if any, of not having a cable.
Personally on a normal desktop PC whenever I have used wireless keyboard/mice I have always ended up binning them in the end and going back to wired. I only use a wireless keyboard for setting up cabinets, but thats a different issue where there is a clear advantage.
But maybe I am missing some benefit so comments welcome.