With the recent cold that covered the United States I once again was fighting to keep my washing machine line from freezing. Typically I just unscrew the connection a bit and let it drip until the hard freeze is over. Its the only line that doesn't run through a relatively climate controlled wall or crawl space and I can't access the line to do much with it as it runs through a portion of crawl space that was bricked over a few decades ago (I live in a REALLY old house). My loosening of the connection to allow it to drip created something of a mess when the pressure increased and it sprayed harder than a drip.

So what I want to do is put a 3/4" coupling with a diverter between the water line and the washing machine's hot and cold feed lines. Something like what might be used for a refrigerator water line. That way I could run the small diverter water line down the washign machine's drain line with a more controlled drip directly down the drain. Anyone happen to know if there is such a thing as a 3/4 coupling that has a water line diverter?

Something like this, a garden hose wye?

your post is confusing.
how can the pressure increase?
when you open anything to let it drip, it just drips.
pressure to that line can not increase all by itself ?
if it is frozen, it will not drip .if it is partially frozen then the water running past the semi-blocked portion of pipe will melt the ice and within a few seconds you will have full flow.