Wordpress XMLRPC advice needed

I've worked with WordPress' XMLRPC system before (using a WP-Hive installation with multiple separate blogs similar to a WPMU set-up). The new approach you're using is definitely simpler and easier to implement (I tried the 5ubliminal one as well the first time).

Whether or not you can distinguish between separate blogs in a MU installation depends entirely on how you build your handler function. You can build it to distinguish the separate blogs, to only function on specific blogs, or to treat the entire system as a single WordPress site. It's all up to you.

By "handler function" I mean a custom function you define to handle XMLRPC requests that call a specific, custom method (not necessarily the default WordPress methods). For example, I use XMLRPC in all my plug-ins to report back installation progress and errors -each plug-in makes an XMLRPC call to a custom handler (method) on my server.

Yes, you can retrieve and post to custom fields.

Absolutely writing a plug-in is the way to go. The only other options are to change core files (BAD idea) or to build it into your theme, in which case it could ONLY be used on MU sites using that theme. Build it as a site-wide MU plug-in that can be controlled on a site-by-site basis by the global admin.

Wordpress XMLRPC offers various functionalities which can be harvested easily. I have used IXR_Library to parse the XML requests/responses. Currently with very small piece of code i can easily posts, fetch, edit and delete Posts in Wordpress based blogs either self hosted or on wordpress.com sites.

http://www.hurricanesoftwares.com/wordpress-xmlrpc-posting-content-from-outside-wordpress-admin-panel/ (reference)

• When you have multiple blogs hosted via MU you will need site ID of all those blogs which will become the first parameter for $params (in our case 0 should be replaced with site_id).
• In the reference i gave above you will see the option to fetch and post to all created custom fields (unfortunately, you can't create custom fields on the fly from my script)
• You are welcome to write a WP plugin to do all of this, be my guest and let me know if you need my help. I have used the same technique to post to blogger, tumblr, Wordpress and Posterous using their API's. I hope this helps.