Lockscreen and progress bar

[jbv at souslelogo.com]

Hi list,

I have a main loop that does a lot of things, like resizing images,
precessing imagedata, creating fields and groups, etc.
On top of the loop I have added "lock screen" to speed things up, and
also because I don't want users to see what is going on, only the final
layout when the loop is over.
However, while the loop is running, I would like to have a progress bar
and a message such as "step 1/20" etc.

How can I handle that ? I took a look at callbacks, but unless I missed
something, it seems limited to players.

Thank you in advance.
jbv