I poked around and couldn't figure out how to pass max heap size to cvm through runmidlet, maybe there is a cleaner way but I ended up using the full command line for the Exec property in my googlemaps.desktop file, notice the -Xmx parameter which now sets max heap size to 24m instead of default 12:

With this value I couldn't trigger the low memory condition which used to occur fairly soon after enabling traffic and zooming in/out several times. In theory this may help to speed up the app somewhat as the garbage collector doesn't have to run full sweeps as often.