Is there a way of linking a custom field to any entry of your website. I've found this add on http://devot-ee.com/add-ons/entry-layout but it's not compatible with matrix. Thanks Geoff
To me it sounds like you might be defining your content too broadly (on a 'page' level instead of on a more atomic 'content' level). However, we still don't know enough to say whether this is the case, and you may just prefer to keep with the page paradigm anyway, so here are some options depending on your use case:
Instead of matrix rows, use entries in a separate channel and then use template logic to pull in those entries on the appropriate URL.
As above but integrated with Shortcode add-on so that the author can decide where the extra entries appear instead of hardcoding it in template.
Take a look at Content Elements add-on. This add-on looks impressive but scares me a lot... (I think you're better off breaking up your content into small tightly defined chunks instead).
Also, since the Entry Layout add-on seems to do exactly what you want anyway, it may be worth enquiring as to whether Matrix is in fact supported - many add-on developers are over cautious about claiming compatibility if it hasn't been tested... or it could be a simple oversight, or it could also be the case that Devot:ee added that compatibility feature after this add-on was originally uploaded.
I suggest taking a look at the Better Pages add-on. It allows you to choose between different page "types" or "layouts", and will hide/show different fields based on which layout is currently selected for that particular page/entry.
Or, you could simply add instructions for your client to the Matrix field: "(only applies to the XX page)". That solution is quick, easy, and free.