Absorption of Topical Magnesium
Homeopathic magnesium lotions don't* contain magnesium, so it seems a little irrelevant whether or not magnesium can be absorbed through the skin. However, magnesium ions can definitely be absorbed through the skin. There's no reason why that should reduce muscle pain(especially because ions would disperse throughout the bloodstream etc etc), but magnesium can be absorbed through the skin fairly efficiently.

*Footnote: they do, but not on purpose. If correctly prepared they would contain zero atoms of magnesium(depending on what 'strength' they are), but that kind of purity is impossible.