Is it possible to slice up an image for coding in something OTHER than boxes/rectangles?
I have a map I need to slice up and have certain regions linked to, without having to upload 150+ images for it to be functional

any ideas?

Yes, why "slice" it at all? If you want clickable regions, use an image map. I don't believe you can have other than rectangular regions, but given enough regions you can make an arbitrarily good approximation of any shape.

In most cases, there is no obvious gain by "slicing" an image.

The clickable regions of an area map can be rectangular, circular, or arbitrary polygons. See also AREA - Image Map Region in our HTML 4 reference.