personal / custom rss feeds?

[gijs]

I'd like to offer my members the option to add a personal rss-feed-block.

There should be an 'empty' block, saying something like this:
"enter a RSS-feed here:"
[address-FIELD] <submit-button>

How difficult would this be to realise? I understand that depending on the amount of members using this, there might be hundreds (thousands) of different rss-feeds in the database. (each shown only to one or a few member(s))

(I use SMF/TP as a collective blog and like to offer members this way more reasons to come back frequently.)