I often hear onegaishimasu used in reply to someone as they begin, however, there is a "different" intonation that seems to mean to me, "Yes, please lets do...thanks for asking" ... kinda and then thank you (in whatever language is appropriate) after the practice. This often happens quickly and informally depending on the situation, etc.
I do think the word implies an "agreement" or yakusoku that saying please in English does not often carry with it. I like the idea and custom of this understood agreement.
Good discussion, thanks.