Hello David & All,
I haven't fed Terramycin in syrup in the last two decades.  When I did I fed
the normal amount you would feed the hive if you were dusting three times as
per directions in a gallon of syrup. I have had trouble getting the
Terramycin to disolve in the syrup at times  when mixing a 55 gallon drum at
a time.  I felt a better exact dose was had by dusting so I went back to
dusting..  The syrup/terra  needs to be protected from direct sunlight and
taken by the bees in short order. . Do not use a boardman feeder in direct
sunlight when feeding Terra in syrup.
I see no problems with your idea but in my opinion dusting would work as
well.  I did however only feed the terra/syrup in one application instead of
three trips to the hive with the dustings.  Sorry but I can't remember the
exact syrup to terra dosage.  I was feeding quite a bit of Erythromycin back
then for European foulbrood which I gave in syrup also.  I don't know of any
beekeepers feeding either in syrup these days. Maybe other Bee-L list
members can help you with  better mixing directions. I believe (from memory)
one pound of TM 25 would treat 50-100 hives. The mix was never very exact by
instructions.  Its been so long I can't remember if we put one pound to a 55
gallon drum or a half pound to a drum. Sorry. I have had several reports of
beekeepers finding AFB in hives they treated with terramycin last fall so
keep a sharp lookout even with your method.
Bob Harrison
Odessa, Missouri