In this scheme, the interest computation doesn't do compounding. So users will have an incentive to spam the network with transactions compounding manually.
For example, suppose Alice has 1000 BitUSD, the current yield is 10% and Alice expects rate of network income to remain the same.
Then Alice can wait to the end of the year and get 100 BitUSD in one interest-claiming transaction. Or Alice can claim interest in six months, increasing her balance to 1050 BitUSD, then in six more months, claim interest on that 1050 BitUSD balance which brings her to 1102.50. The extra 2.50 BitUSD Alice extracts far exceeds the transaction fee. Finding the optimum timing of interest claims is left as an exercise to the reader.
We can fix this relatively easily. Let r = TOTAL_ACCUMULATED_FEES / (BIT_ASSET_SUPPLY - TOTAL_ACCUMULATED_FEES), then the interest should be ACCOUNT_BALANCE * pow(r, PERCENT_OF_YEAR_HELD).
My formula takes compounding into account, so Alice can no longer extract a risk-free profit with the above strategy (the result of claiming interest twice in a year is a balance approximately one tx fee less than the result of claiming interest once in a year.)