I'll guess Reverdy Johnson (Mary Surratt's lawyer) I can see he was politically active but cannot find a statement about presidency.
Ok, Judge David Davis wasnt one of the lawyers but he was involved in deciding whether it was to be a military trial, and he was in line for succession for being President (due to vacancy of a Vice Pres when Garfield was killed)
“The honest man, tho' e'er sae poor,
Is king o' men for a' that” Robert Burns