I’m looking for info on setting up an SMTP connector to accept all mail sent to the server (MX20) for a domain, hold it and forward it onto the primary Exchange server (MX10).
Start with a simple working config, refer http://support.microsoft.com/kb/319426 for a good guide.
I'm not sure why you specify "hold it and forward it" - as this is the default behaviour of an Exchange SMTP connector: if the network link that the exchange connector uses is down then the email is queued until the link comes back up again. This assumes that your SMTP links come up for a sufficient amount of time every few hours or less.
You didn't say what version of exchange you are running, if Exchange 2007, then refer to this queues tutorial which gives some examples of how to use PowerShell to manage queues: