simultaneous disabling / enabling of all LTM objects

Well you could just use 'all' in place of the Virtual Server name in the command as shown below but I would advise thinking this through a bit more thoroughly before you do that as it may well have unintended results. 

modify ltm virtual all disabled  

Imagine you have three Virtual Servers in the following states:

VS1 - Disabled

VS2 - Enabled

VS3 - Enabled

You run the command above and that sets all three Virtual Servers to a Disabled state which fulfils the criteria you have asked for but what happens when you run the command to enable all the Virtual Servers again? There is no consideration made for VS1 which was initially disabled - all three just change state from disabled to enabled. VS1 may well have been disabled for a very good reason but now it is potentially back up and accepting traffic again which could be very bad indeed.