I’ve been running a 100mm SID on my Scott Scale for years, and I’ve been really impressed by the difference in capability and comfort compared to the 130mm setup on my Spark (though I realize that’s not an apples-to-apples comparison since the Spark is a full-suspension bike). It makes me wonder whether adding another 10–20mm of travel on the hardtail would be worth it. The extra length would definitely affect the geometry and rider position—so I’m curious about others’ experiences. Is the added front suspension travel on a hardtail worth it, and how has the geometry change impacted the bike’s feel—better or worse?
The way I read that first sentence is that you feel the Scale has more capability and comfort than the Spark. Is that what you mean or do I have it backward? Normally, the longer travel FS bike would be considered to be more comfortable and capable, and putting a longer fork on the Scale would make it somewhat more capable and comfortable due to the increased travel and slight slackening of the HTA, and maybe slightly less efficient. IMO, the effect on rider position would be pretty negligible. Adding 10mm will raise the bars only ~6mm at the sag point. So it would be a little different, but whether that’s better worse is totally subjective. Hard to say how to value that though wrt being worth it or not. There are HTs that are designed for and come with 150mm forks, so people do ride them.
Welcome to the forum, @skepticalempiricist! 
I’ve done this a handful of times, and I’ve ridden my current hardtail with a rigid fork, 100mm, 110, and 120mm.
BIKECAD is a great tool for looking at how your bike’s geometry will change with different fork lengths (or any changes, really).
The most noticeable difference I’ve found that can be hard to deal with when lengthening your fork is the slacker seat tube angle.
Quite a few hardtails still have somewhat slacker STAs, and you can quickly get into a slightly compromised position if you’re not careful with that.
The other obvious changes will be a higher BB, higher stack height (I’m a bit sensitive to this), and shorter reach, but you might be able to remedy the stack & reach with a different angle/length stem.
The slacker HTA will likely be welcomed, but if you’re having to add a longer stem to make up for the shorter reach, your steering will likely feel a bit slower than it did before.
Most bikes like the Scale are designed to be lightweight, and “fast” with little/no suspension in the front and rear. Adding 20mm of travel up front won’t likely transform the bike, but might be welcome depending on your riding style, the terrain you ride, and your intentions for the bike.
There’s going to be a solid balance for the geometry of your bike, and if it was designed for a 100mm fork, a 110 might be better than a 120. If you really want to upgrade, a 110 with 35mm stantions (I’m not sure if they make a SID like that or not) or a 110mm 34SL would likely feel quite a bit different as they’re more stiff in the right ways while offering more travel and feeling a bit more plush.
Going that route will add weight to the bike, though, and the only real upside to a hardtail is that they can be built up really lightweight..
Personally, I’d keep your Scale really light and ride your Spark when you want the extra comfort, control, grip, and capabilities. Your Scale will never feel like your Spark!
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FWIW: Back of the envelope: 10mm increase with a 1150 wheelbase changes angles ~0.5 degree. For a saddle height of 75cm, 10mm moves the saddle back ~6.5mm wrt the BB. 20mm would double those. For me, those would be minor changes.
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I tried that on a hardtail around 15 years ago and I hated the end result. The bike felt, hmmm, not great. In my case the fork was adjustable from 80 mm to 120 mm, and it only felt right when I hit around 100 mm. Back then I knew next-to-nothing about bike geometry, and I was taken aback by the difference.
Some people may be less sensitive to the changes or even would welcome the changes in geometry.
Another point to keep in mind: in my experience, different type of forks are made to do different things well. I have a Fox Factory SC on my hardtail, and this puppy soaks up small bumps as if it were candy. But I have to be careful with bigger hits.
So even from that perspective, your bike will feel different with a trail fork. (Not saying better or worse, just different.)