Glue logic is simple to implement with a MACH (or other CPLD). I've also done parallel to SPI serial conversion in a MACH (for the speed).

You have to learn to use the manufacturers tools (editor and fitter) which are usually a free download, and get familiar with the programming language (usually Abel or VHDL).

To program a MACH you either need a device programmer ($$) or a propriatory cable ($) or a home-built cable (information on the Lattice web site http://www.latticesemi.com/) and free "ispVM" download software.

To program a device in circuit from an 8051 you need to read about "ispVM embedded software". As for programming "on the fly", bear in mind that CPLDs have similar limitations to flash memory with regard to programming speed and number of cycles.

To do DSP operations, you need a real FPGA, rather than a MACH. Try the Xylinx or Altera web sites.

Good luck!