In the book 68000 Microcomputer Experiments by Alan D. Wilcox that I recently mentioned in a blog post, there is a chapter that covers using a cross-assembler to build 68000 code and upload it to the Motorola ECB. The assembler used for the examples is the Quelo 68000 cross-assembler.
A couple FORTH implementations for the 68000 that I came across mentioned that they were written for the Quelo assembler as well.
Quelo was a company based in Seattle, WA that offered a commercial cross-assembler for the 68000 that ran on CP/M and MS-DOS, around 1984. A Google search located a copy of the Quelo assembler for MS-DOS as well as a manual.
I was able to run the assembler under Linux using the dosbox MS-DOS emulator program (It likely runs under Windows 10 as well, but in my experience dosbox is often even more compatible with MS-DOS than recent versions of Windows are).
I used the small program example that was in the Wilcox book and was able to assemble it, generate an S record file, and upload and run it on my TS2 computer.
Assembling my larger calculator program generated some errors which looked trivial to fix (e.g. the syntax for PC relative addressing was different).
It looks like quite a sophisticated cross-assembler which includes macro support and a separate linker as well as some utilities like a cross-reference tool and utility for splitting files into odd and even ROMs.
All of the files I used as well as building instructions can be found here:
A Google search for Quelo found a number of references to CP/M and Motorola 68K software including the ad above from the November 1983 issue of PC Magazine. The manual says it was licensed for a single user. One source referred to the CP/M version as being public domain; possibly they made the CP/M version of the assembler free when MS-DOS became more popular, but as far as I can tell the source code is not available anywhere and is likely lost.
It was fun to be able to run a program from 1983 on a modern computer, but for most of my 68000 cross-assembler needs I think I will stick to the VASM assembler which runs natively on Linux and is still being actively maintained.