I wouldn't say anything if you just play it, but if you have been heavily involved in some mod project, putting that on your resume can only help you. Employers won't hire you on just that basis but they generally like to see signs of creativity like this, and in a marginal decision where they have a couple of equally qualified people, they will go for the more interesting person.
I was recently asked about my "mod development" for a research scientist job (in DSP/control algorithms) and a quant analyst (finance) role. Neither of the people knew much about games but seemed very interested in learning more about what I did.