If only he had! I suppose it could be as you suggest, but the context does not seem to me to fit a past tense indicative. Just a few lines earlier he even write

If only he had! I suppose it could be as you suggest, but the context does not seem to me to fit a past tense indicative. Just a few lines earlier he even writes ‘Thou … art alive still, while thy book doth live,’ and the previous sentence matches the subjunctive + conditional structure (‘if I thought … I should commit’) that I think he is using in the disputed sentence (‘though thou hadst … I would not seek’).