Employees must be paid for meetings and seminars. See exact rules at the US Department of Labor web site. If the meeting or seminar is not in the office it is OK to pay a lower rate provided the employees have agreed to the lower rate in writing and the hourly rate is at least the minimum wage.
Your staff might not like that but on the other hand the extra time will likely put them into an overtime situation which will likely balance out the lower rate.
That being said I think it is foolish to pay a lower rate for meetings as you'll just get all your staff angry which is not exactly the frame of mind you want them in for an office meeting.
Bottom line: Don't step over dollars to pick up pennies.
Note: When a hygienist, for example, is taking CE to meet a requirement and you are not involved with scheduling, etc. you do not need to pay for that time. Also some states, such as California, have have additional rules for missing lunches, etc. so always check with your lawyer or a local HR pro to ensure you are compliant with your own state’s regulations.