No, I didn't forget about trying to put together an application to modify dimension leader landings. It just took a while to really check out the limitations and possibilities. The exercise turned out to be an opportunity to learn more about some of the features and limitations of AutoLISP programming, dimension variables, and the AutoCAD entity database. Actually, it seems to be quite complicated. There are lots of settings, but as in your case maybe not the exact one you are seeking.
Unfortunately the original LISP application did not fully work out after more testing and analysis. While the original application is able to modify the dimension leader landing and text gap length independently, the change is only temporary. After modification, if I select a grip on the dimension, modify a dimension property through the Properties window, or open the drawing with RECOVER the dimension leader landing reverts back to the style defaults. The only way I found to get around the reversion is to explode the dimension after modification. So that to me is not a good solution, and not worth posting.
So after more analysis and experimentation on my end, here is the less elegant work around: Override the dimension gap for a given dimension and draw an extension to the leader landing. So you could do the following:
Determine the total length for the leader landing and text gap that you want.
Divide the total length by 3 times the dimension scale (New Dimension Gap = Total Length/(3 x Dimension Scale). Type DIMSCALE to find current scale if needed.
Type DIMOVERRIDE on the command line.
At prompt to "Enter dimension variable name to override ...", type DIMGAP.
Type in the New Dimension Gap calculated above. Hit Enter key twice.
Select the dimension(s) that you want to modify.
Reposition dimension leader(s).
Draw an extension line(s) to extend the leading landing(s) to the desired position next to dimension text.
The procedure above is correct for linear/rotated and aligned dimensions, but would need to be modified for other dimension types or if the dimension scale for a given dimension has been overriden. Also the DIMASZ variable does have an affect on the leader landing length but not for linear/rotated and aligned dimensions.
So with all of that in mind I did put together an AutoLISP application that in essence leads you through the process off extending the leader landing for an individual dimension (see attached). It is questionable whether this is really that much help, as in some cases you my have to further adjust the dimension leader to get it display the way you want. But it does seem to work fine in most instances. While I have gone through testing the application, there still may be some instances where something does not work correctly. Like I said, lots of variables!
Anyway, if interested download the attached dgo.lsp file, open the .lsp file with Notepad to read more about it prior to using it, and load it through the Load/Unload Applications dialog box (type APPLOAD at the command line). Type DGO at the command line to execute application. Test it out and see what you think.