Cocoa insert text on webpage?

I have the need to interact with website forms on my mac. These forms don't work well with any piece of software out there (way beyond 1Password and iCloud Keychain). I found that I could retrieve much of this information from the user keychain and local user account info, and some "stock" info I could put in a file somewhere (or declare statically in a block of code). The idea is to click a button, and then the forms get "magically" filled out from the info stated.

I have a short list of websites I needs to access, but I'm wondering about the best approach here. I need to first get myself setup when accessing Safari, and then Chrome later. I could use bash, python, or objective-C (or Safari Extensions if needed) to accomplish this.

The users are OK with having to push a button in the web browser to make it happen - to doesn't have to be completely automatic.

Any ideas on this one? Extensions for Safari don't seem to have any stated Objective-C library access. Not sure where to go with Chrome..