Complex Tables and the TH element

Greetings,

I am developing processes for converting complex financial documents to
valid, accessible HTML.

Traditionally, I have marked up tables where the first column is marked up
as a TH and set scope="row" for accessibility (and semantic meaning).

However, some of the documents I am working with are 5 column Statements
of Financial Position where the first two columns are estimated numbers,
followed by a column containing the row labels, and finishing with two
columns of actual numbers.

A quick perusal of the HTML 4.0 recommendation does not preclude me from
marking up the cells in column 3 as TH with scope="row". I am wondering if
anyone else has marked up documents in this manner, or can elighten me if
this is an abuse of HTML 4.0, or is aware that this presents a significant
barrier to assistive technologies?

kind regards
Terrence Wood.