Variable substitution in channel entry

Hi, I’m looking for some general guidance.

I need to allow a site Editor the ability to add a tag variable to a Channel entry via the Channel Publish section e.g. {myvariable}, within the entry body text, so that when a site page is accessed, {myvariable} is substituted with query string-specific data.

So if a site page is accessed via the URL

http://mydomain.com/category/mypage?data=foobar

{myvariable} is replaced with ‘foobar’ throughout the Channel entry.

Can anyone give me some idea of where I would need to start with this? And what stage of the parse order I would need to hook in to to make this work?

Any and all advice on this would be appreciated.

Thanks in advance.