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Regarding the $BN$-pairs approach, I must say I don't recall details, having done very little work on these things in past 15 years. In a nutshell, one cannot hope for "real" apartments, etc., so one instead looks at amalgams of parabolic subgroups. Instead of a definition, let me giev you a toy example, $GL_4(2)$ and its Borel subgroup $B$ (taken to be the upper-traingular matrices, say). Then you have "minimal parabolics" $P_i$, i.e. subgroups generated by $B$ and $e_{i+1,i}$, for $i=1,2,3$ (here $e_{ij}$ denotes the matrix with 1 at position $ij$ and on the main diagonals, and 0s elsewhere). Then, you get maximal parabolics, $P_{ij}$, generated by $P_i\cup P_j$. This is what is called a rank 3 amalgam (as you have 3 minimal parabolics).

Your geometry then consists of cosets of $B$, $P_i$'s, $P_{ij}$'s in the whole group and in each other. The amalgam is now the set-theoretic union of $B$, $P_i$'s, $P_{ij}$'s, and you can study its universal completion, i.e. the biggest group where is can be embedded into. By tweaking the groups which can arise as $B$, $P_i$'s, $P_{ij}$'s, one covers more cases than buildings, and tries to stay away from infinite universal completions for ranks at least 3.
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There has been a lot of work done on various generalizations of the concept of the building, to apply them to sporadic groups. These generalizations are variously known as diagram geometries, chamber systems, etc. Names like G.Stroth, S.Smith, M.Ronan, A. Delgado, D. Goldschmidt, B. Stellmacher, etc. spring to mind. There is an "elementary" book on diagram geometries by A.Pasini (a review of the latter is here.) There is a series of books by A.A.Ivanov (some of them are jointly with S.Shpectorov) developing a theory of this sort to deal with a majority of sporadics.

Indeed, one needs a weakening of the classical buildings to cover sporadics. Instead of starting from a weak BN-pair, one can weaken Tits' axioms from his "Local approach to buildings" to develop a theory dealing with sporadics. E.g. Witt designs for Mathieu groups (already from 1938) are extensions (in certain well-defined way) of the affine plane of order 3 and of the projective plane of order 4. Similar things can be done with $HS$, $Suz$, Fischer's sporadic groups, $He$, $McL$, $Co_3$, $Co_2$, and $BM$. (E.g. --- cannot resist citing myself here: the 3-transposition graph for $Fi_{22}$ can be characterized as the extension of the polar space for $U_6(2)$.) This appears to work when the underlying combinatorics is not too complicated (and the corresponding permutation representation has low rank).

PS. IMHO, Aschbacher sometimes tends to ignore prior work, re-inventing the wheel in different terminology.