Using J(2,H), gives determinant preserving group SL(2,H)=SO(5,1) in Coll(HP^2)=Str_0(J(3,H))=SL(3,H)=SU*(6). Like the SO(3,1) case, where one represents the momentum vectors as bi-spinors in J(2,C), a given vector is lightlike if and only if its determinant is zero. Hence, in the quaternion case, SL(2,H)=SO(5,1) preserves the lightlike property of each quaternionic bi-spinor.
If one considers a Freudenthal triple system (FTS) over J(3,H), its automorphism group Aut(F(3,H))=SO*(12) contains SU*(6), as the collineation subgroup over each HP^2 component. The FTS contains two copies of HP^2, considered as electric and magnetic projective planes in N=2, D=4 symmetric SUGRA for dyonic extremal black holes.
Moving up to the octonionic case, we have SL(2,H)=SO(9,1) in Coll(OP^2)=E6(-26). This gives FTS automorphism group Aut(F(3,O))=E7(-25) which contains an SO(2,10) subgroup. Your SO(2,4) is recovered if we restrict to a complex sub-algebra. Hence, in the octonion case we essentially have an AdS_11, which upon reduction to a complex subalgebra, we recover your AdS_5.
Extending the FTS to 57-dimensions gives E8(-24) with an SO(3,11) subgroup.
For division algebras, we have the inclusions:
R: SO(4,3)->SO(3,2)->SO(2,1)
C: SO(5,3)->SO(4,2)->SO(3,1)
H: SO(7,3)->SO(6,2)->SO(5,1)
O: SO(11,3)->SO(10,2)->SO(9,1)
In the split division algebra cases:
Cs: SO(4,4)->SO(3,3)->SO(2,2)
Hs: SO(5,5)->SO(4,4)->SO(3,3)
Os: SO(7,7)->SO(6,6)->SO(5,5)
There is a super Yang-Mills theory formulated with SO(11,3) gauge symmetry, as given by Sezgin in hep-th/9703123. Bars extended this to supergravity in hep-th/9704054. Also recall that Lisi has interest in SO(11,3) based on Nesti and Percacci's GraviGUT theory [link:arxiv.org/abs/0909.4537]arXiv:0909.4537 [hep-th][/link]. With the insight from extremal black holes, it is tempting to conjecture the SO(11,3) super Yang-Mills theory is contained in a larger theory with E8(-24) symmetry. An (11,3)-signature theory would be a 14-dimensional generalization of M-theory in D=11.