The way the core of TST is made does not allow for this. It builds based upon what the split file is like (along with other settings not currently in the Mac OS version but intended to come).
Although this could be a feature, generally I use OBS settings to fit it as I need it and I know that TST will always be the same window size. Is there a reason that you cannot just use the properties in OBS (or X-Split) to get the desired results?
Eric, has the FooterHeight and FooterWidth from the Window section of the ini file been implemented yet? Those add additional height and width, respectively, to the window after the size of the window is determined from the splits, which has the effect of stretching the window. Also, because this is a setting, it is persistent across sessions.