The process you requested is possible, I have reauthored both the install iso and the 2nd CD of the set. I use the 2nd CD, the live system CD, with a 2.4.19 kernel to support IEEE-1394 Firewire devices not supported by 2.4.18 kernel. I can provide some thoughts and/or directions below. The process here is definitely for an advanced user. A lot of CDR's can be wasted.
You can find the directions for the authoring process in the isolinux directory on the install CD, filename README.TXT. If the installation is the problem area then a new kernel for 2.4.19 can be built and installed as the boot kernel by replacing the kernels/bare.i directory contents with your new files. The System.map.gz is just a gzipped System.map file. If you wish to create your own directory, then please look at the isolinux/isolinux.cfg file. I believe it would require modifications for your boot options to work. I would highly suggest that you start your 2.4.19 build using the 2.4.18 bare.i config file. Then verify or change what is needed to resolve the compatiblity issues. Be sure to change what you require to kernel built options and do not use any modules. If you include your 2.4.19 tar ball on the CD then you can extract it after the install is complete and transfer your config file. Then after rebooting, build the kernel system under 8.1, and include the modules that you would like to have built. You should then be able to install the modules into the proper driectory.
The Slack 8.1 iso requires 700 MByte CD at present. You will probably require more space than available. You can elliminate KDE or GNome from the slackware directory to achieve the space. If you desire both then you can either install one of them later or mount the original install CD for the install process. If you use the later then you need to update the kernel manually after the install is complete.
Good luck and I hope the info will be of some assistance.