See Situation 85 in this article:
When one recites a bracha, he is not only reciting a bracha--he is also performing, at the very least, a Mitzvah D’Rabbanan. Thus, assuming that one is not making a bracha MiD’Oraysa (such as bentsching or perhaps Birchas HaTorah)--if he makes 100 brachos a day, he is also fulfilling 100 Mitzvos D’Rabbanan!
My inference from this quote - Birkat Hamazon is D'oraita, and since you had a s'eudah, you are required to say it, so there's no violation, here. There is also a requirement to say 100 Brachot per day, and the Bracha rishona on the desert is D'Rabbanan. You are making the bracha rishona on the desert for the purpose of another mitzvah, namely, getting to the goal of 100 mitzvoth.
The vague part within this evaluation is - how does the term "tzricha" - necessary - apply, here? While there is a mitzvah to say 100 Brachot daily, I don't think it is a requirement to do it. Therefore, according to that logic, making the separate bracha, perhaps is "eina tzricha".
I am inclined to say CYLOR, but, something tells me that your rav might tell you CMY (Contact Mi Yodeya), so you'll create recursion she'eina tzricha :-)