Is there a way I can add a transaction to my own memory pool without broadcasting it to the network?  Imagine I am mining my own transactions and want them kept secret from the network unless they are in a block.  (For example, a time-locked commitment that triggers another event which I want to make sure can only happen AFTER my tx is in a block.)

As far as I can figure, the only way to do this is to mine the transaction myself.  And right now, the only way I can think to do that is to run two nodes where one is a bridge "blocks only" node and is the sole node that the other (mining) node connects to.  Then I can add transactions to the mining node manually.  But then it won't know about any other transactions on the network that I might want to also include in the block I am mining.

Is there a way to accomplish this?  To have my transaction in my block template without broadcasting it?