I've managed to make it work in this way:
* deploy the zimlet
* as zimbra user, create the _dev directory
* always as zimbra user, move the zimlet directory in the _dev folder
# su - zimbra
$ mkdir zimlets-deployed/_dev
That works for me, and I have uncompressed zimlet files and compressed zimbra core files in my browser.
$ mv zimlets-deployed/com_zimbra_zimlet zimlets-deployed/_dev
Every page reload takes uncompressed zimlet files without caching them, so you can view your changes immediately with a normal page size.
It's important to do that with zimbra user, because it sets right permissions on files. If you do that as root, it may not works.
That's not works with jsp files, for which I have to do
every time I make a change. Let me no know if someone knows a simpler method to reload them.
$ zmcontrol restart
PS: move zimlet folder in the original position again, makes the zimlet disappear. Seems that lost the LDAP deploy. I've fixed undeploying and redeploying it again. Maybe coping the original in _dev folder instead of moving is enough and don't breaks the LDAP deploy, but I've to test it...