PowerPC machines can definitely run Linux and join in distributed computing as a node. I’ve used distcc / crossdev tools to distribute compiling of gcc builds across different architectures in a cluster (ppc + x86_64). The typical use case of this is to get my Mac Pro to do the grunt work for my ppc Macs though, not the other way around.
If you're looking for pure processing power, the PowerPC Macs will lag behind massively and may not even have anything to contribute to an existing multi-core, multi-threaded cluster's processing capability. Where a single Intel system could fire off 16 threads (or more) at 4Ghz+, even the mightiest Quad G5 is limited to 4 threads (1 thread per core) at 2.5Ghz and an energy draw of something like 300 watts/hour or more. Then there are architectural constraints, from memory and bus speeds and SATA throughput, all of which are massively bottlenecked compared to a modern system.
The answer is "Yes, you can. But is it worth the energy consumption?" If you were mining bitcoins with a bunch of G5, then you'd hope to have a massive (pre-installed) solar panel grid just to sustain the cost of running compared to modern GPU based parallel computing clusters.