flatgrinder, I'm not familiar with 18-8 stainless but think it's similar to the 316 stainless you referenced. Both austenitic stainless' that will have little or no response to heat treat when it comes to hardening. I think both will work harden a bit though. For a frame lock, liner lock, etc. your best choice when it comes to stainless, IMHO, would be 416, a martensitic stainless steel that can be hardened and tempered. Its great to work with too, much better than 300 series IMHO. Other than that, like Paul mentioned, 440C would work good too but would be much tougher to machine than 416. And, like you said, Ti would work fine, but, in any form ............ its a bear to machine compared with 416, and more costly.
EDIT TO ADD: I just re-read your post and if I understand correctly you're going to just make the lock bar from steel and insert it into the G10 scales correct ? Not much machining to be done other than maybe some draft where it slides into the scale (like a dovetail slot?), or some screw holes?. Ti, 416, 440C, or even the same material as the blade stock you plan to use but tempered back to ~ 46 ish +/- < /> And, if you do decide to use Ti, I would at least flame harden the lock face so its not sticky
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