I've created a custom field on the gl account for report purposes (specialized reports are needed for bank financials vs tax financials) With NS canned financial reports, can I use that custom field to drive the layout?

tl;dr No, you can't. Longer answer, If you have a custom segment you'll be able to view columns by this, if that's what you're going for. If you're talking about adding the field to the financial layout section criteria, I don't believe there's any fields you can add regardless of how it's configured. The unavailability of custom fields in all areas of reports is one of the major drawbacks of the reports framework. Your field definitely will be available in saved searches and SuiteAnalytics Workbooks, but then you have to rebuild the logic for the IS, BS, or TB report format. If it fits your use case and you can swing it with your accounting folks, you could use some part of the account number to drive this or just select the specific account names (but then you'll have to maintain this). Wish I had better news. Hope that helps!

Ok, thanks @James Morris. I figured this was the case but I needed confirmation that I wasn't missing something.