My two penneth worth is this:
The problem with wargamers is that they only tend to put their SYW infantry battlions into line formation rather than their battalion line formations into a longer contiguous line of battalions without intervals because the rules don't give the correct fighting bonuses for doing it. (It's one of the reasons a lot of SYW games end up looking 'Napoleonic' in nature and you see infantry in battalion squares – a formation they were not overly trained in adopting because it was rendered redundant by the use of contiguous lines of battalions).
A contiguous line of units in line has only two flanks. Cavalry charging frontally at any point along the line should therefore treat it's target as being in square – an infantry unit out in the blue in line shouldn't because it's the supporting units directly to either side that secure it as such.
Rules that do not highlight this fact need changing – it will encourage infantry to form up in cumbersome long SYW style lines moving as one – linear tactics as opposed to impulse tactics of the Napoleonic period. (Impulse tactics is where units, or small groups of units, are given specific missions to carry out; they weren't used much in the SYW).
Banning SYW battalion square is cut throat but, never the less, a thing to consider.
Even when forced to face in two directions (to front and rear) at the same time, when cavalry got behind the line, infantry battalions in contiguous lines were pretty safe.
Nosworthy's books on battle tactics, Anatomy of Victory, and Battle Tactics of Napoleon and His Enemies, are a very good references for this kind of thing. The former is possibly the best book on SYW tactics and the the latter is good because it often contrasts the differences between linear and impulse tactics and has a very good section on cavalry charges.
BTW, Charles Grant hates the term Old School and we all had a good giggle at his expense at the last meeting of the League of Gentlemen Wargamers where Charles umpired a very large game using The Wargame rules written by his father. The term 'Old School' has now been replaced by the term 'Traditional' in that circle.