Outside of installing a tremel-no, Is there anyway to semi-block a recessed vintage trem so you can only push it down? Or will I have to install a full block?
You can put wood between the trem block and the inside of the cavity (look at the trem cavity when doing a pullback, that's the motion you want to restrict). When people block a trem, they usually put it on the side of the trem facing the bottom of the guitar - you would want to add a much skinnier block to the side of the trem facing the neck, if that makes sense.
The Floyd Rose Trem Stop is the easiest and cheapest way to go....super easy and works great. I sometimes put a little piece of felt on the screw where it touches the block so it doesn't "click" when it touches.