My recommendation for this issue would be compiling php5 manually and configuring it for only *.php5 files in Apache configuration. Then you'll be able to develop the site in PHP5 uploading php files with .php5 extension.
Is this a request that I could create a ticket for to get done?
Lunarpages is not responsible for handling software setups on your server (except one free script install in Managed Hosting Intense addon). However, we can assist with this request for $75/hour assistance fee mentioned above by other forum member.