That's just lazy on the payroll provider's part. I've never considered a catchup is anything other than an increase in the deferral limit for those who are age 50 and over. If you are, then your limit is $24,000 (and if you're not, then your limit is $18,000). So, my issue would be that the notion of a separate election being required because they arbitrarily decided to curtail someone's deferrals before they met their statutory limit is unacceptable.
At the end of the day, this is a reflection of free market forces at work. If you can do this and retain your clients, then good for you. I won't try it :-)
Good Luck!