I want to clarify what I think the relevant issues are here. I think that there are two ways of approaching the issue. Meno, Yekke, & Sam are approaching it from one angle, and I (and the Sefarim I looked at) are approaching it from another.
The two possible issues are:
1. The reason that you can’t continue eating once you make a bracha achrona is that the bracha achrona provides a” “hefseik”. So the question here is: Does the “al hamichya” in this case exempt the rice and thereby create a “hefseik”?
2. Even if it doesn’t exempt the rice, can one say that a bracha rishona that no longer applies to the food on which it was made can still apply to another food (in this case a food that one specifically had in mind when he made the original bracha)?
I was approaching the issue from the first angle, and Meno & Yekke2 were approaching it from the second angle (as far as I can tell – they can feel free to correct me if I’m wrong).
If the answer to 1. is that the al hamichya exempts the rice in this situation, then 2. becomes irrelevant since he clearly needs a new bracha.
If it’s a safek if the al hamichya exempts the rice, then “safek brachos l’hakel” and he is not allowed to make a new bracha, but it is also better if he doesn’t eat the rice. However, in that case, if we knew for sure that the answer to #2 is that the bracha of mezonos was terminated and can no longer be “chal” on anything, then he can certainly make a new mezonos.