So my mash temp, by the recipe, was 149 F. I missed it by a few degrees, at 144 F. Can I just let it sit longer, or would I have to find a way to increase the temp? This was in a Rubbermaid cooler MLT, so the only way to increase the temp would be to add more, hotter water.
You could do either.
If you leave it at 144, you should definitely let it sit longer. Even a difference of 5 degrees F. can make a significant impact on how long it takes starch to break down (it could take twice as long, or more). It will definitely still convert at 144, and will give you a more fermentable wort (and the resulting beer will be drier). Look for clear wort vs. cloudy wort as a good indication of starch conversion. Or, ideally, test a sample with iodine to confirm starch breakdown.
I worry getting much below 145 though because then you're pushing the limits on dipping below the gelatinization point of barley starches. If the starches don't gelatinize, the enzymes can't reach them and then you have real problems.
If you choose to adjust the heat, adding boiling water to a mash is perfectly acceptable, in fact I'd guess we've all done it at some point. This would get you within spec for the recipe and also speed up conversion.
144 is at the low end of the saccharification range, so you've still got conversion. You might have a little less dextrin, and a little more fermentables than the recipe intended, but if you're using US/CA/AU 2 row, you've probably have so much enzyme in there that 144 or 149 will give same result.