Here's how I solved the problem (without paying partner / without dev licence):
1. There was no way to use "Notes" (without dev licence)
2. I chose to use Sales Order Comments, and set "Print on Pick Sheet" to TRUE... and how I did this is below:
As per my comment above, simply creating a web service for Page 67 and attempting to create a Comment, was failing with error:
Error (OData): The Sales Comment Line already exists. Identification fields and values: Document Type='Quote',No.='',Document Line No.='0',Line No.='10000'
I was getting that error, regardless of the params passed...
I also noticed that *if* a manual comment was entered in NAV, and subsequently you would try to enter another comment via webservice - there was no error... but this observation didn't help me. I had a need to enter comments via web services, to new orders also entered via web services.
What i did - that solved this issue is to create a new Page of type "Card" for NAV Table 44 (Sales Comment Line) in "Microsoft Dynamics NAV Development Environment".

I gave it an object ID of 50081, named it SalesOrderComment, made it available as a web service...
Now, using ODATA to create comments with this new service, there are none of the issues I encountered above..
Note that although I'm able to create a PAGE, whenever I try to create or view any C/AL code or CodeUnit, I get permission error (and I know we do not have dev licence). But we do have the "Microsoft Dynamics NAV Development Environment" installed on the NAV server.

Hope this helps someone out there