Serial DB9 to Serial 3.5mm over HDMI cabling
Using an HDMI cable for rs232 isn't a problem. A cable is a cable is a cable. As long as you are sure it won't be plugged into an HDMI device by mistake (typically not a problem in one off hobby projects). And length may be an issue, as forgiving as rs232 is. A few feet isn't a problem though.

Making your own adapters is the only practical solution you have. Get a db9 dongle and a HDMI connector. If wiring small pins is an issue, there are prewired db9 pigtails and something like this 

 

The second adapter would be HDMI to a 3.5mm Jack or pigtail.