Turns out, North Trail is not the same thing as North Grove Loop, and is most likely a horse trail not yet open for the season.
The following day, after a trip down into the foothills via another extremely skinny, windy road, made scarier by the fact that the outer lane was being repaired after collapse (Gil only just made it), we decided we were ready to take this walking business seriously, with a 6km uphill hike from our camp ground to Marble Falls. After following a clear path for several hundred metres, we suddenly found ourselves climbing a very steep hill, grasping at roots and rocks, with Brad vocalising bitterly what our park entrance fee was for if not clearly marked trails and a few signs. Fortunately, the prior days experience, together with the lack of evidence of other hikers having taken the trail (we'd seen multiple groups set off earlier), led us to return not long into our climb and once back on the definite path we spotted a not very obvious sign the to correct direction. The trail travelled through several different types of terrain as we gained roughly 400 metres in elevation and was littered with an assortment of lizards and hopefully harmless snakes and, given the much lower elevation, was snow free and rather sunny (Brad will now take more bottles of water for himself on such hikes, rather than drinking from questionable water sources). The falls were well worth the 12km round trip, which the following day reminded us that yes, we do have leg muscles, with a huge amount of water quite literally thundering down, amplified by melting snow.
We still haven't seen a single bear!