I don't know if it's the quickest way or even if it would work for you, but one can rotate the canvas a different angles via the Rotate View Tool.
Then pressing the shift key should constrain the stroke to that angle.
The Rotate View Tool shares the same slot in the toolbox as the Hand Tool or keyboard shortcut R
Thank you R_Kelly! Though it is still a bit tricky judging the angle as you rotate the page, your workaround is actually a bit quicker than having to rotate each line individually.
This allows me to see the line as it develops in place and I also don't necessarily need to create a new layer for each new brush stroke, if I work carefully. Very close to placing the ruler on the work space before drawing a line!
Or touch down at one end of the stroke, and hold down the Shift key while you touch down at the other end.
Or paint your line on a new layer, and use Free Transform to rotate and position it.
Buy Corel Painter X3 program, does exacly what you want to do with brush confined to paint along a perspective grid, controlled by your choice of setup. See their web site for feature demo (new in X3 version).