Displaying a message telling to contact server administrator will be the last thing to do. First of all I'll try one of this solutions for automatic config creation:
Create 2-3 sandboxed .htaccess configs in subfolders
- During plugin activation test configs one by one with simulated remote AJAX test
- Start from the best general settings config subfolder
- Check for AJAX proxy script calls and image, style etc. files access in this folder
- After finding successful(unblocking and not crashing) config, save selected .htaccess file to the folder containing AJAX proxy PHP file
- If none of the configs (or built-in wordpress AJAX script - not very reliable) is functional display an error telling to contact server admin to allow htaccess for given folder
OR
- Check loaded modules with PHP
- add SecRuleRemoveById id only if mod_security2 is present to prevent basic mod_security crashing