Hi Goldbear
Well I think the only way you can do mahamudra without bending your knees is to have your legs straight out in front and touch your toes. You could have a rolled up sock under your perineum where your heel would be (if you were doing mahamudra.
Not sure if that would work. You are leaning forward and doing the mudras and bandhas the same. But its still not mahamudra!
But, I reckon, an easier alternative, would be to do Yoni Mudra Khumbaka. Everything is similar except you are not leaning forward.
It's like me, I have rubbish shoulder mobility so I can't do the easier version of the headstand or chakrasana. But I say oh well, there are plenty more powerful practices to do instead.
