I confirm this. Currently too much sanitizing is happening from within the backend. I’ll see what can be done (while still abiding by the wordpress rules).

For custom fields there is a hidden field, for the standard rsvp fields: you either use them or not. The only exception is the #_SEATS, which normally should be used. Btw: if you don’t want to be used (because it is always 1), just put the min and max seats to book at 1, then the field will be hidden automatically. Otherwise there’s still the option of putting the css for hiding a field in your theme css file (or read up on eme.css in the doc concerning stylesheets).
Like I said: I need to analyze what the requirements are for sanitizing input so I can mimic the regular post-sanitizing here (which seems to be very open …). Currently it uses wp_kses_post, which only allows what wp allows (but you can allow more using kses-filters).