I don't use that log file for loading the history. I use SHRALIAS and a global history and only load the history (from SHRALIAS dump file) at login. You *could* use the history log for loading; only the most recent entries will wind up in the history (as much as will fit). But loading that file can be very slow. Loading my history.log, about 5MB, takes 4 minutes. It takes so long because after the list is full, TCC must (one by one) discard entries at one end to make room for entries at the other end.
Perhaps Rex would consider an enhancement to HISTORY /R, namely start loading at a position in the file such that the remainder of the file is guaranteed to fit in the current history size (that's all you get anyway!). That wouldn't be hard to do, set the file pointer to FILE_END - HIST_SIZE, read forward to an EOL, then start loading history lines. I suppose it would be equally easy in a plugin.