Definitely. As much as we can complain about how "complicated" it is to make this happen with the game coding, it is nonetheless possible even if it is just as flavour text or along those lines. I'd be delighted to see elbows or knees happen in the standup, however rare it would be (and it is rare enough even in real fights).
Yes, it would take some work, but you can test is extensively first, and then adjust in once you roll it out into the game. Make the difference quite small, if punches are mandatory for standup striking, kicking is a strong addition, but not manadatory. Elbows should add the advantage of maybe half of kicks, to standup striking. I think that's a good mark to aim for, in terms of power.
Knees, like mentioned above, should also be an advantage in standup striking. Roughly, at the same strength as elbows in my opinion. ESPECIALLY when instigating clinch, defending from clinch initiation, or breaking from clinch. These are distances just between clinch and standup range. But knees do have a role in standup striking, although much smaller than kicks, but that's a given.
Striking wise (not clinch), maybe, a fighter with 120 punches and 120 elbows and 120 striking def, should be an equal match to a fighter with 135 punches and 120 striking def. That sounds reasonable to me.
Taller fighters should have a slightly easier time with elbows at striking range, but they should work for everybody. Because in reality, they do work for everybody. Shorter fighters have better elbows at clinch range.
In the long run, I would like to see a box or a an option called FLASHY MOVES, these are high energy, high risk moves (flying knees, spinning elbows) that work depending on the fighters energy level, morale, confidence, intelligence, and experience.
I would think these things mostly work when fighters are tired if when you have an energy advantage. I feel like shorter fighters, in general, have a higher ability to jump and land flying knees, so I would give them an advantage in this. I would also give shorter fighters an advantage in spinning moves, like spinning elbows, because a smaller body can spin faster and easier without losing balance.
The main attraction with knees + elbows should be the clinch. But they should add a little something to standup striking as well. This would make them more desirable and create more game diversity.