How would I enable SIP without using recovery mode?

No. Won't buy you much anyway because the entire Catalina operating system is on a read-only APFS partition. Apple does not intend users altering it.

Apple has the synthetic.conf feature in Catalina (and Mojave) that allows you virtually symlink a local folder to a designated operating system location.

man synthetic.conf

Why don't you want to boot into the Recovery volume to turn it off? It's a simple process.

Are you sure it's turned off? Launch Terminal and enter this command; csrutil status

I would think when upgrading to Catalina it would have been turned on.

Also without SIP you're at extreme risk of any adware and malware and even the possibly being hacked (if someone sees your Mac as a worthy target).

The Terminal command is csrutil enable but you cannot do it from a mounted disk, that's why you need Recovery or another disk with a system on it.

Bummer, can't even turn off SIP if the user wants to.

So how did you disable it?