Glad you mentioned this. We have talked about this topic internally too in the past days. Unfortunately the value returned is in PAL and there isn't an easy way to show the USD reward. We had a way to get the USD amount for standard Steem Payout, but now that we have more than one token is not so easy to get the total value.

Probably this can be done for the 'estimated' value on pending payouts (it will take more code and some more API calls). But the real challenge is to get the correct value for post already paid because I don't think there is an API to view the historical price of the steem-engine tokens. Getting the USD payout for a post paid 1 month ago will be really difficult.