AutoCAD multi-leader objects (command MLEADER) allow to use background text masks for annotation, but this mask cannot be preset - neither with mleader styles nor with any other standard method.
With the free utility MMleader you can use preset text masks in multileaders. Download this utility from www.cadstudio.cz/download and load it into AutoCAD (e.g. permanently using the briefcase in APPLOAD).
The new command MMLEADER works the same way as the standard MLEADER, it just prompts before its first use (in a given session) for the mask offset (overshoot) and its background color. These values can be also preset using two LISP variables - e.g.:
(setq _MML_offset 2.0)
(setq _MML_color 10)
An alternative command MMLEADEROPT always prompts for these mask parameters, on any launch.