Does RESPA prohibit quoting Owner's Title Insurance on the GFE for a refinance?
I know the FAQ state that a loan originator MAY enter "NA" or "Not Applicable", but does it prohibit entering an actual value?

from http://dictionary.law.com/Default.aspx?selected=1229,

may - v. a choice to act or not, or a promise of a possibility, as distinguished from "shall," which makes it imperative. 2) in statutes, and sometimes in contracts, the word "may" must be read in context to determine if it means an act is optional or mandatory, for it may be an imperative. The same careful analysis must be made of the word "shall." Non-lawyers tend to see the word "may" and think they have a choice or are excused from complying with some statutory provision or regulation.

In RESPA context IMHO I would interpret "may" to mean it's optional, not that it is prohibited.

The second part of the question is:

If OTI is quoted on the GFE and the borrower does not purchse a policy, can you still use the quoted value in the tolerance section on page 3 of the HUD?

Unquestionably, no. See this reference from one of the RESPA Roundups:

III. Consumer Does Not Use Service
Finally, we get the following question frequently: If a service that was listed on the GFE was not purchased, what should go into the borrower’s column on Page 2 of the HUD-1 and on the comparison chart on Page 3 of the HUD-1?
If the consumer did not purchase a service that was listed on the GFE (usually owner’s title) there should be nothing entered in that line on Page 2 of the HUD-1 and the estimate of the charge should not appear on the comparison chart on Page 3 of the HUD-1.

Thank you