It happens that you won't see a command line windows in SAPGUI, e.g. when you are in a modal popup window. There is an ample trick to send an arbitrary command to the SAPGUI.
If you want to send the "/H" command to activate the debugger, you need an arbitrary text file with the following content.
[Function] Command=/H Type=SystemCommand
Then you simple drag the file over the popup window and drop it there, et voila!
There is a possibility to set a watchpoint on an internal table to halt the debugger whenever a row of the table is inserted or deleted (but not when a row is simply modified!).
For every internal table there is an implicit adminsitrative header line, that takes the name of the table with a preceding asterisk (*).
E.g. data: itab type table of t000.<a href="#ref01">ref01</a>
The administrative header for table itab is then *itab, on which you may set a watch-point.
Note: The administrative header will be available only when the table is created, i.e. the after some row has been appended to it the first time.