As the person mainly responsible for the planning part of Subsurface I have to disappoint you: It is unlikely that this feature will be implemented (at least any time soon).
Of course, there is the "recreational mode" that extends the duration of the last manually entered segment to the maximum time the diver can stay at that depth without incurring any mandatory stops (a better version of NDL) but there is no way to say "I want to do a maximum of n minutes of stops/ascent time". It wouldn't be hard to implement but the problem with such requests is that they would increase the complexity of the already very baroque user interface of the planner. You would need at least one field where you can say that 15 is your magical number of minutes. But imagine we would scratch your itch. Then the next user would ask for a feature like "what is the maximal time I can stay at given depth with the deepest stop being 15m?" or anything the like. There would be another box for the 15m. And so on. Part of the problem is that there could be many variations of it and each would ask for additional UI elements. So, unfortunately, no, we are not going to add another value box for the request of a single user. (Things might be different if we were convinced that this particular feature would be useful to a significant fraction of our user base).
That said, even if not automatic, you can easily find that time manually: Plan you dive as usual, then select the last waypoint in the profile graph with the mouse, then use the cursor right key to increase bottom time until you are happy with your ascent time (checking total runtime and end of bottom time in the planner notes on the right bottom). The "plan variations" feature might also be handy here. Plus of course that fact that the Subsurface planner instantly updates the plan when you change parameters of your dive.
The "optimal gases" part is even more difficult: First there is the question "optimal in what respect? minimising ascent time? minimising gas cost? minimising narcotisation? taking into account cylinder sizes? maybe with contingency planning?" Another problem with likely more "correct answers" than technical divers using the Subsurface planner.
A related but possibly not obvious feature: In the gas/cylinder table, you can enter a "*" in the MOD or the "MND" field. That automatically computes the "best mix" where the O2 fraction is computed such that the max depth is the MOD of the gas while the He fraction is computed to have a prescribed END at the bottom part of the dive. But currently, this only works for bottom mixes.
Another problem with optimising the gas for the shortest ascent time is that what would require calculation several plans (one for each trial gas) which likely would compromise the "realtime reaction to parameter changes" that we are quite proud of.