Since I was unable to find a way to get Matlab to read these older files, and was unwilling to manually save all the worksheets to .csv, I resorted to (shudder) Applescript for the task. Veering off the Matlab topic, but in case any other Mac users out there encounter these Matlab/Excel incompatibilities, here's the script I used for the task:
-- excel2csv.applescript
--
-- Created by Kelly Kearney on 2011-08-31.
--
-- This script processes all .xls files in the directory and saves
-- all worksheets within the files to .csv files, named <file>.<sheet>.csv
-- User chooses a directory (b/c I can't figure out how to do this
-- via input variables)
tell application "Finder"
set inputDir to choose folder with prompt "Select a folder"
set filelist to files in inputDir whose name ends with ".xls"
end tell
-- Convert all worsheets of Excel files to .csv files
repeat with x from 1 to count of filelist
set f to item x of filelist
set csvPaths to {}
tell application "Microsoft Excel"
open f
set theSheets to worksheets of active workbook
set workbookName to name of active workbook
if workbookName ends with ".xls" then set workbookName to text 1 thru -5 of workbookName
repeat with aSheet in theSheets
set thisPath to (inputDir as string) & workbookName & "." & name of aSheet & ".csv"
set end of csvPaths to thisPath
save aSheet in thisPath as CSV file format
end repeat
close active workbook without saving
end tell
end repeat