Unfortunately, boundaries like that aren't possible by default in SAMP ( you can only define a rectangle as a bounding box). Altus made a custom boundary system that made what you suggested possible. But again, to draw the boundaries we're limited by SAMP again, and we solved it by drawing lines using gangzones (the same ones that we use to draw the spawnzones). So, as we're drawing the boundaries with rectangles that we made to look as lines, we're limited to only having bounds parallel to the x and y axis.
So, if I understand this correctly, you're saying that Altus' boundary system would allow boundaries that are not parallel to the X or Y axes, but that the system for drawing boundaries is incapable of displaying them?
Assuming you wouldn't hit a limit on the number of gangzones, why couldn't you draw small, square gangzones to act as "pixels" and draw whatever shapes you want with them? Once you can draw pixels, you can use a simple algorithm like
Bresenham's to draw any line you want. Hell, you could even do B-spline curves to make mapping the boundaries impressively smooth, though that would be overkill. I don't know how the gangzones are drawn or tracked, so I'm not sure how (or if) it would have effects on performance, but I don't think drawing with Bresenham's would be too bad.
You could even optimize Bresenham's by drawing the long, rectangular zones you're currently using for multiple pixels in line parallel to an axis, when possible (so in
this picture, you'd draw 5 short, disconnected lines parallel to the X axis of lengths 2, 2, 3, 2, 2 pixels instead of all 11 pixels individually, but if you had a 45° line, you'd still draw it with your individual "pixels"). That way, you'd avoid whatever overhead might come from having more small gangzones than you need (if that overhead is even significant).
If, before investing time in implementing Bresenham's, you want to test how the game behaves with possibly hundreds or thousands of small gangzones, I think you could pretty easily convert the current non-oblique system to using entirely pixels or just draw a
fuckton of pixels in one spot and see what happens.