Summary and Info:

Recently started using read-only mount of '/usr'. Had to implement hooks to remount it as read/write before installation and remount it back to read-only after installation of packages. For instance:

[Trigger]
Operation = Install
Operation = Upgrade
Operation = Remove
Type = Package
Target = *

[Action]
Description = Remounting '/usr' read/write...
Depends = util-linux
When = PreTransaction
Exec = '/usr/local/sbin/usr-remount' rw
AbortOnFail

and

[Trigger]
Operation = Install
Operation = Upgrade
Operation = Remove
Type = Package
Target = *

[Action]
Description = Remounting '/usr' read-only...
Depends = util-linux
When = PostTransaction
Exec = '/usr/local/sbin/usr-remount' ro

Works very well except for

CheckSpace

in '/etc/pacman.conf'. It always gives an error that there is no space available due to '/usr' still being mounted read-only at this stage. I think 'CheckSpace' should either be reimplemented or (if not possible) a hook with similar functionality should come by default with the 'pacman' package, so that one could still use it by putting it after remount to read/write one.