I think this question is based on a false premise: that all of Pope's couplets were meant to be exact rhymes. Some rhymes in An Essay on Criticism might have been exact in Pope's time, but not today: there is some discussion of this in the article "Mother Hubbard’s Bone, Alexander Pope’s Tea", by Allan Metcalf, and in the comments to the Language Log post "Who would not weep, if E. B. White were he?"
But there are also some rhymes that seem unlikely to have been exact even at the time when the poem was written. According to "Language, and Pronunciation as shown by Rhymes", from the Introduction by E.E. Morris to Pope's Essay on Man:
All Pope's rhymes cannot be defended on the ground of change in the pronunciation. 'God' could not have rhymed with 'abode,' i. 125, with 'wood,' iii. 155, and with 'road', iv. 331
(p. xxx, 1900)
In the Essay on Criticism, we also observe the following rhymes that seem dubious to my eyes:
-
Some ne'er advance a judgment of their own, / But catch the spreading notion of the town
In other places, Pope rhymes own with known and none, and rhymes none with sun and upon. If we assume that Pope pronounced each word a single way, and only used exact rhymes, then it seems we have to conclude that Pope pronounced own, town, known, none, sun, upon with the exact same vowel. That seems fairly unlikely, since these words did not originally all have the same vowel and they don't all have the same vowel today.
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As shades more sweetly recommend the light,
So modest plainness sets off sprightly wit
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Both must alike from Heav'n derive their light, / These born to judge, as well as those to write
If we assume that Pope used a single pronunciation for each word, and only used exact rhymes, then apparently he must have used the exact same vowel in wit and write. That seems unlikely. I suppose you could argue that Pope pronounced light as "lit" in one couplet and as "lite" in the other, but that also seems implausible to me.
It looks like An Essay on Criticism was written and published in the early 18th century. During this time period, pronunciation was not so different from present-day English pronunciation. According to "Early Modern English: Phonology", by Alex Bergs and Laurel Brinton (2012), from Historical Linguistics of English,
ME /ɔː/ as in boat or no had, by virtue of the GVS, moved to /oː/. In the late 16th and 17th centuries, it was joined by ME /ɔu/ as in blow or know. Whether the vowel resulting from this merger was monophthongal or diphthongal is debatable and without doubt varied from one dialect to another (see Section 5.1.2). From the 18th century onwards, the diphthongal realization /oʊ/ prevailed in the standard (and became Received Pronunciation /əʊ/ around 1920).
(p. 596)
A [sound change] affected short /ɔ ~ ɒ/, which was lengthened at the same time and in the same environments as /a ~ æ/, i.e. before voiceless fricatives. Examples include loss, off, and cloth. Rather than creating a new phoneme, in the late 17th century the sound merged with /ɒː/ (which resulted from the monophthongization of ME /aʊ/ in the middle of that century). For a while, long and short versions coexisted side by side, but again the long ones were partly stigmatized. In contrast to the lengthened /a ~ æ/, short /ɒ/ was eventually restored before voiceless fricatives (except for some speakers of southern dialects).
(p. 602)
So in Pope's time, the word boast would have been pronounced something like [boːst] or [boʊst], and the word lost would have been pronounced something like [lɒst], [lɒːst], or [lɔːst]. It's unlikely that these words had exactly the same vowel, but [oː~oʊ] and [ɒː~ɔː] are fairly close and apparently this was sufficient for the words to count as a rhyme according to Pope's criteria.
tchrist's answer to the question Was the pronunciation of “symmetry” different in the past? provides an example of Pope rhyming the word toad with abroad, which seems similar, although it's true that the retention of /ɔː/ in present-day English in the word broad and its derivatives is irregular and so I guess it might be possible to argue that Pope might have used a now-lost variant pronunciation of abroad with [oː~oʊ].