Can a floating capacitor terminal be used to collect electrons?
It will be probably much easier without the capacitor. Shining the e-beam on the steady potential electrode will generate a much more stable current reading using a conventional transimpedance amp.
The problem with the capacitor terminal is that it will charge; and eventually this charge will deflect the e-beam from hitting the electrode.
Another issue is that you will generate secondary electrons emission losses from your target, so if you want to measure the beam current accurately, the target should have a hollow geometry with the beam entering through a hole. That way, stray emission will be recaptured.
Depending on the beam intensity, target heating can become an issue.