My 32 bit laptop is a Thinkpad T42 from 2005 which has a functioning CDROM, and which can run Slackware15 stable 32bit install OKish, so I haven't tried any of this but:

My first thought: How about using a current computer to run qemu then mounting the Lenny iso as an image and installing to a qemu hard drive? Then dd the hard drive image to your 32bit target. (That might need access to a hard drive caddy depending on how you can boot the 32bit target machine, so a 'hardware regress' I suppose).

My second thought: If target machine is bootable from a more recent live linux, try a debootstrap install of a minimal Lenny with networking (assuming you can connect target machine to a network, I'm guessing with a cable rather than wifi). Reboot and install more software as required.