@rcombs ,
The iFixit Chemical Dispensing Pen is made of polypropylene which has a broad range of chemical compatibility.
[link|https://www.calpaclab.com/polypropylene-chemical-compatibility-chart/?srsltid=AfmBOoqIRVeKdmkC6J92XBdENb3lyFMrW_L12W3sttaS4092VUoNlKk9|Here is a more comprehensive list for your reference|new_window=true].
With that said, diluted detergents should be just fine to keep in the pen. Concentrated detergents may embrittle the pen and cause cracking eventually.
As for flux, tacky flux like the iFixit No-Clean Flux would not work, but liquid flux like Kester 951 may be a good choice.
