I'm thinking that if you rolled the leg potion of your pants down to make gaiters, you'd either have to hold them up around your calves with an elastic loop, or allow them to gather around your ankle, it would require something added to make them work properly.
This would be especially prevalent with the lightweight fabrics desirable in hiking pants.
If I were in your shoes, I'd start by shopping for zip-offs that fit well, and had an appropriate shorts-mode inseam, and add calf zips if needed after the fact. It's not hard to cut off belt loops, cargo pockets and flaps from existing clothing to trim disliked features and weight. Fit and length are the two hardest parameters to match.
I'd start there because zip-offs are essentially the same price as plain pants, and the hardest part has been done for you.
If that failed, I personally would look at something like the Mtn Hardware Mesa pants. I had a pair a few years ago and they were light and quite durable. They met their fate in an unfortunate incident that involved me, my motorcycle and an oncoming driver taking his half out of the middle, but I walked away relatively unscathed, considering I was wearing hiking pants in a slide/roll down a dirt road.
This is fun to discuss, and think outside the box, but light zipoffs and dirty-girl gaiters seem like the most effecient combo possible.
For comparison, here's what I'm wearing:
They weigh 10.5oz, and the shorts come to about 3/4" above my knee when standing. I have about a 31" inseam.
If you were super intent on lightening up your clothes, removing the mesh brief, cargo pockets(all external pocket) and belt loops could turn these into about what I think you had in mind in the OP.