As written by Stephen before:
There is no vectorized way to set the value of sub-fields of struct arrays.
[ax(1:3).UserData.data] = ...
cannot work, because [ax(1:3).UserData] creates a comma separated list, but you cannot obtain a subfield of such a list by the dot operator.
A loop is a simple and efficient solution. If you have spent a half day for searching a vectorized version, you are a victim of the "premature optimization" programming pattern. In many cases the programming and debug time exceeds the run time massively, such that the total time to solve a problem does not profit from too tricky improvements in lines of code, which do not consume a remarkably part of the total processing time. So start with writing dull loops and if the program is running and tested successfully, use the profiler to find the most time consuming parts. Optimize these bottlenecks only.