Yes, you could. You'd implement a program to take C code and emit an equivalent Lisp program (i.e., parse C and emit the tree into Lisp), then compile the Lisp program. The annotations and care the parser/semantic analysis code uses will provide the semantics of C.
This will, however, not "play" like C without a great deal of work. You will be dragging an entire Lisp VM around and have to shake all of that out to actually produce something as lean as a C program at the end. You're probably best suited to take the Lisp compiler and build a "independant code" generator to generate binaries that don't rely on Lisp. But I'm waving my hands, you'd need to talk to the SBCL or CCL development teams there.
That doesn't sound significantly easier than what we've done to implement C using Truffle. I'm not sure I buy this argument that what we've done could have been done trivially using Lisp macros.
Honestly, the front end would be about as hard, unquestionably! But the Lisp ecosystem is already in play, whereas you seem to have had to build that ecosystem. That is to say, it's not a major effort to scrape through the Lisp systems to understand what is going on, whereas your ecosystem with Graal etc required a great deal of work, it seems to be. Would that be correct? Or was it pretty plug and play after you had JVM bytecode?
Why compile to lisp in the first place? Build a nanopass framework (https://www.google.com/url?sa=t&source=web&rct=j&url=http://...) that acts on sexprs, and then emit the thing to a target of your choice. The advantage is that Lisp is really good at transforming sexprs, and macros make writing the infrastructure for that sort of compiler just that much easier.
This will, however, not "play" like C without a great deal of work. You will be dragging an entire Lisp VM around and have to shake all of that out to actually produce something as lean as a C program at the end. You're probably best suited to take the Lisp compiler and build a "independant code" generator to generate binaries that don't rely on Lisp. But I'm waving my hands, you'd need to talk to the SBCL or CCL development teams there.