What is a sensible and safe FLOAT charge rate for a 14 V, 12 AH sealed
lead acid battery?

I am not clear on the question, so I will expand on the answer and hope to
cover the question. First, you mention a 14 volt battery rather than a 12
volt battery. The float voltage would be 16.1 volts. The float charge rate
would be equal to the self discharge rate of the battery , which is
typically .001 times the battery capacity, or .012 amps (12 ma).

The sealed lead acid battery can be left on float charge indefinitely.
However, that is assuming that there is no load on the battery. Putting a
load on complicates things.

Colin