orware The "integration with other databases" feature you describe has been requested before, more than once, although the ability to use a previous field in the same or another Machform to control the choices in a dropdown menu has probably been requested more often. I heartily second this feature recommendation; it would be invaluable at my community college.
I have requested the ability to limit access to individual forms based on LDAP authentication myself, as soon as this new feature came out. Again, I second this request.
The ability to limit the number of times a radio button field can be selected has already been added to Machform, although, disappointingly, you can only set a limit for the field as a whole that applies to each choice. So if you always have the same number of seats for each choice it works; but if you want to allow choice A to be chosen 25 times; choice B, 30 times; and choice C, 15, you are out of luck.