The simple answer to this question is "no". Hanslick was not known to
Wagner when he wrote his first Prose Draft of 'Die Meistersinger' in
July 1845. The character who in the libretto (of 1862) would be given
the name of Sextus Beckmesser is a caricature of music-critics in
general and it is beyond doubt that one of the music-critics whom Wagner
had in mind when he wrote the libretto was Eduard Hanslick. 

It is widely believed, however, that Beckmesser was a caricature of
Hanslick alone. There are two reasons for this widespread but erroneous
belief. The first of them is that, in the second Prose Draft of October-
November 1861, Wagner gave the name Veit Hanslich to the Marker and Town
Clerk. This was a private joke of which he soon tired, however, and in
the poem or libretto written in January 1862 he gave this character the
name of Beckmesser. The second reason is Wagner's account of a reading
of his poem in Vienna in November 1862 ('My Life' pages 703-4).
According to this autobiographical account, Wagner believed that
Hanslick was in some discomfort at this reading and friends of Wagner
who were present got the impression (according to Wagner; his account
is not corroborated) that Hanslick had seen himself as Wagner's target. 

In late 1846 there appeared in the 'Allgemeine Wiener Musikzeitung' a
number of references to Richard Wagner by a young music critic, Eduard
Hanslick. The young man hailed Wagner as "the greatest living dramatic
talent". He sent Wagner his enthusiastic review of 'Tannhäuser', for
which Wagner thanked him in a long letter of 1 January 1847. This was
the beginning of a friendship that eventually collapsed under the weight
of differences of opinion about musical aesthetics. Hanslick became
increasingly critical of Wagner, who began to regard the critic if not
as an enemy at least as no longer a friend. Hence the joking renaming of
the Marker as "Veit Hanslich" in the second Prose Draft of 'Die
Meistersinger'. 

On closer examination there is no reason to believe that Hanslick saw
the poem of 'Die Meistersinger' (in which the character was called
Beckmesser, of course) as a personal attack. Indeed there is nothing to
indicate that he knew about "Veit Hanslich". Not even in the account of
the Viennese incident in Hanslick's memoirs (see Spencer's compilation,
'Wagner Remembered', pages 135-138). His supposed reaction to the poem
is a myth of Wagner's invention. 

The myth has been given a new lease of life by Barry Millington, who has
argued that Beckmesser is an anti-Semitic caricature.  The reason for
Wagner to introduce the anti-Semitic references that Millington has
ingeniously decoded is, we are told, that Beckmesser is a caricature of
Hanslick, whom Wagner believed to be of Jewish descent. Those who wish
to read more about this complicated theory are referred to the articles
listed under the answer to Question D above.