Hiya and welcome back,
I can take this under advisement as a requested feature, but the main thing Rejecto does is create a language model that incorporates the rejection features and has the rejecting elements added to the language model’s probability calculation, so there is no getting around the requirement to recalculate the lm’s probability model even if you start with a completed one. It also has to check and make sure that you aren’t already using one of the rejection phonemes in your real model and if so remove that phoneme from the rejection model, meaning that a premade .dic would also still need processing.
Not saying there is no way, just that it isn’t trivial and it won’t vastly cut down on processing time.
If you are seeing unpleasantly slow generation for a dynamic model, maybe you want to look at this tip I wrote up which had a suggestion at the end for avoiding any repeated use of the fallback pronunciation generation technique (i.e. the slow one):