Thanks for getting in touch Stephen. Wow, big change for your feet, running in luna sandals! Here are a collection of my thoughts about your options:
Socks will definitely help. So if it's no trouble to wear them, I'd do it. Socks don't guarantee blister prevention, but they'll go a decent way towards it. They'll also protect your feet from abrasion (sand, grit etc).
ENGO Blister Patches
These might work too well. If it's just a discrete area that's blistering under the ball of your foot, and you can cover this with a small or large oval, fine. But if you need wider protection in the form of a rectangle patch, we might be reducing friction too far over too large an area. More than ever, now you're in a sandal, you rely on high-ish friction levels under your foot to keep your foot stuck and relatively stationary on the shoe. Plus there's the hold provided by the upper, which has now been cut down to a few straps. So if there is significantly less friction holding the sole of your foot, I fear your foot will slide forward too easily and you'll get trouble from the pressure exerted by the straps.
Taping could be worth a try. I don't think taping reduces friction levels too much (nor do socks for that matter, but they do a little bit plus they're an extra interface which serves as protection). It could all end in tears though if the tape comes loose and folds over itself and becomes an irritant - so it definitely has its limitations. But assuming you need broad protection across the ball of the foot, I think taping might serve you better than ENGO in sandals (don't ask me which one Stephen - have a read of this if you haven't already).
Keep these up your sleeve for blister treatment, should it come to it. Here's what you need to know about donut pads.
Luna Trail Sandals
To wrap up
Right now, I favour socks. However, if that's not enough, add taping to the equation. If that didn't do the trick, add an ENGO rectangle patch and hope the straps don't annoy too much (I'm really keen to know how that would go now). And if you still manage to blister, then donut pads (plus the appropriate dressing).