I'll try out your fix when I get home tonight.

Other way to fix
 

My bad: it's not the iconv binary that misses in the initrd for syslinux shipped in 14.1 to work, but gconv, shipped in packages glibc and glic-solibs.

In Slint installer (that already ships gconv-modules and libraries in /usr/lib{,64)/gconv associated to encodings of PO files for util-linux that are not encoded in UTF-8) I had just to add the library IBM850.so as that is the default in mtools. I checked, that works and the USB stick is usable so I'd tend to go that way for Slint.

But I assume that if Patrick wants to fix the issue in genuine Slackware installer for Slakware 14.1 and/or Slackware current, he could prefer the other way (stated in post #15) to avoid adding gconv stuff to the installer. I will use the same fix in Slint then.