filtering pages by tags or custom fields (meta_key)

I want to show the content of a given page (or summary through custom fields) on my homepage.
This page should be selected randomly from a pool of pages with a particular tag or custom field. I think that it is straight forward enough, but I’m not good enough with php to figure it out. I can do this for posts selected from a particular category, but if I could solve this it would save a lot of duplication.

I’ve had a trawl through the codex and forums, but can’t find what I am after. Any pointers would be really appreciated.

Thanks

Nathan