Perspective grid

Is there any reason you can't use a traditional pipeline for this, of the sort used by graphics APIs such as OpenGL and Direct3D? That would probably be the most flexible way to do it.

Because I'm using a 2D library for a 2D game. :P

Suffice it to say that drawing the grid tilted is mainly for show. ;)

I don't know exactly, how you make the grid, but I would represent the grid like in the image
Point D is the bottom of the grid, the vector u is a unit vector that points to the top of the grid. (in my image u isn't a unit vector, it's already u*d)
Point A is the eye point and the red lines are the coordinate system (the origin is in the middle of the picture). The vertical line is the y-axis, the horizontal line is the z-axis.

The coordinates of points on the green grid line can be computed with

P = D + u*t

where t is the distance between P and d. For example:

G = D + u * (1 * d)

where d is the regular distance between the horizontal grid lines.

now we can use the similar triangles ADH and AD'H' to compute the y coordinate of our intersection point D' because the distance D'H' is the y coordinate(in my picture y of D' is negative).

y = x/xD * yD

where:
y = D'H'
x = AH'
yD = DH
xD = AH

Soooo, I hope this can help you and that my English wasn't too bad, so you can understand everything.