Is it possible to add more php to the theme

One of the things that there have been a lot of posts about on this forum is how to use more php code, beyond the limited set. Recently, Juggledad posted a patch to extend which limited php code (beyond what was originally permitted) can be used in a certain area of a template.

Moderator note: Patch 113-06 does not add any more PHP to the theme, it simply allows some of the existing limited PHP to run

Here's my idea. Can that same concept be applied to extend the permitted functions further, even beyond the limited set of allowed php?

Maybe the cleanest way to do it would be to put all the custom functions in a separate file and include them with an include statement such as:

What I like about this approach is that I'm not big on editing theme or WP files. (By using plugins, I haven't had to do that yet.) However, if all I had to do was insert one line of code to include all my custom functions, and I could then eliminate one or more plugins, I'd probably do it.

Maybe as a user group we can jointly figure out how to best do something like this. Any ideas?