What precise subversion are you using? I can do Make Editable without it losing anything... (whether I pre-select or post-select in the copy).
Of course the tool could be enhanced to prompt you to make it editable if the destination cellView (where you click to paste) was readonly (this would really need to be an enhancement CCR though), but I think that would be overkill - better just to figure out why the Make Editable approach is not working for you first (otherwise the exact same problem might occur when the tool itself does a make editable having prompted you).
One situation I can imagine where it might discard your select is when the data on disk is different from the readonly copy you have in memory. In that situation, doing a Make Editable does a refresh from disk (which it must, because otherwise you could clobber data carefully saved by the last person who edited the cellView) - and then any selection in memory would also have to be discarded because the objects in the original cellView have been discarded (even if the actual shapes which were selected are still in the same place when refreshed, the tool has no idea that they're actually the same shapes because the database objects get recreated). I just tried this and it does indeed throw away the selection in that case. Even if the tool prompted you during copy, you'd hit exactly the same problem.
Perhaps that was your problem?
In IC616 you will be able to use the yank and paste commands with pre-selection - so you can pre-select shapes, yank them (into a memory buffer) and then paste them - (a bit like a ctrl-C/ctrl-V combo in tools on Windows). A different paradigm, but that might solve your problem going forward.
To prevent Copy altogether when readonly would need you to wrap the command in another function which checked if the cellView was readonly before you started - you'd then have to change your bindkeys (and probably the menus/toolbars if you cared that much) so that they called this new wrapper. Not sure it's worth it.