Hidden category for multiple blogs

I run several blogs that share the same database, they are all installed on the same server but with seperate wp installs and url’s blog1.com blog2.com etc..

there is one database with all the tables shared with the exception of a seperate options table for each blog wp_options_blog1 wp_options_blog2 etc..

what i would like to do is to be able to exclude some posts from some blogs… after much searching and playing around with plugins and hacks i feel the best solution that wont cause too many problems with future upgrades would perhaps be a plugin that would enable the use of hidden categories and only allow posts from a particular category to be displayed…

what i would like to be able to do is add the following categories _blog1 _blog2 _blog3 with the underscore defining it as a hidden category… when i write a new post it will be placed in 1 OR MORE of those hidden categories as well as 1 OR MORE of the displayed categories.

blog1.com would only display posts that are in at least the _blog1 category and the query filter would be applied at the end of all database queries so that it was applied across all pages on the blog.

i have tried several plugins for making categories (in)visible but they dont handle posts with multiple categories well

my php skills are limited and im sure i will figure it out in the end but if anyone can give me any pointers or has any alternative suggestions i would be most gratefull

xslippy