Yes, simple is a must. If savers cannot transparently see past average interest rates and how much they have earned over the past month, year, and so on, they won't be very interested. After all, it's the savers this would appeal to, not the traders. But isn't the vision for the network that the peg is mainly supported through trading and "arbitrage" activity, not because of a coincidental balance between saver and shorting demand? So, this might not boost BitUSD demand as much in the short term as one might hope. In the longer term, traders will be doing most of the work supporting the peg within tight ranges, so here again the savers are less important in maintaining the peg.
I think if an interest program is implemented, it should be in the main BitUSD, not a separate bitasset. A bitUSD5 would kill off demand for the main BitUSD asset, making it even harder to maintain the peg.
I also worry that even paying interest on the main BitUSD could change people's perception somehow. This changes the entire dynamic. With an interest program, BitUSD would seem less like a twin to the USD and make it harder to eventually maintain a really tight peg.
Finally, consider what would happen if, someday in the future, inflation goes high (which forces nominal interest rates to rise--they will need to be above inflation in equilibrium). Would interest on BitUSD still be competitive if traditional interest rates rise to 10%, 15%, 20%?
Basically what I'm saying is that paying interest seems like only an indirect, blunt way to artificially stimulate demand for BitUSD. It could work in the short term, but it certainly has some drawbacks and could have a weaker-than-expected effect on demand BitUSD since it appeals to savers, not traders. It also could have a number of unintended consequences and weaken the psychological basis for a tight peg.