the only way to circumvent the command line argument length limit is to put fewer data on it. Programs that require huge amounts of data usually allow the user to specify file names to read that data from in addition to specifying said data directory on the command line.
Unfortunately looking at the code for the Univention Directory Manager daemon it seems clear that there’s no way to specify a file name to read the data from. The argument to --set is split into two parts, and the second part is then taken as-is (after re-coding it to UTF-8) as the value (see /usr/share/pyshared/univention/admincli/admin.py, lines 627 and following).
Note that shells offer ways of reading command line arguments from files, e.g. someprogram $(</path/to/file). This won’t work as it’s the shell that reads the file, not the program itself; therefore the file’s size still counts against the argument length limit.