Not in the manual... what you are probably hoping for is by holding down some combination of keys, you only affect movement of the control point in either X or Y direction. Not implemented as far as I can tell.
The custom curves are already "constrained" in a manner. The X axis is a representation of the distribution of values "incoming"... and the output "Y" axis is a proportionality multiplier. There really is no absolute control of input --> output value, the custom curve is really only a shape that influences the rest of the black/gamma/gain behaviour. It is not analogous to curve adjustment in Photoshop at all.
"I always pass on free advice -- its never of any use to me" Oscar Wilde.