Orange and rhubarb is a classic flavour combination. Lemon not so much. I would not substitute. I think they'd be horribly sour and they'd lack the intended flavour.

^Agree…lemon is much too acidic, especially with rhubarb. You need the sweetness of the oj.

Hm... Meyers are half orange, and sweetish, if you have one. Though I've just realized, I buy a specific OJ because it's pretty tart. Fresh squeezed at home tastes like candy to me, and most of the bottled does too, so the others might be right that it's impossible, but if you're up for taking a chance, you could sqeeze the lemon, taste the juice, and add sugar a pinch at a time until it has a flavor you like. Or, if you're not after cake-like sweetness, just go for it. The baking soda should offset the acidity. Or if the lemon is super acidic, add another pinch of soda. It will puff, so don't add too much. Oh, but don't use bottled lemon juice, unless it's fresh squeezed. The stabilized kind is too processed.

Adding some sugar or simple syrup to the lemon juice might work until it tastes right to you. I just think subbing 1 1/2 cups of straight lemon juice sounds mouth puckering with rhubarb. Also, the flavors of orange with rhubarb can be very nice.

It depends on the rhubarb. Sometimes it's insufferable and requires a boatload of sugar, but mostly, the rhubarb I've used is merely quite tart. Baked in muffins would be plenty sweet, I'd think, but again, I'm used to a more sour OJ than many kinds. I figured the juice was was there to activate baking soda. I think Carol is right about watering it down, though, if it's too tart.