It should not be an issue. What is making a problem for you is that you are trying to climb out of the saddle in a gear that is much too low for that technique. Out-of-saddle climbing demands a certain amount of momentum, so that the pedal sort of carries itself over the top. In that low a gear, you will come almost to a stop between downstrokes, so that progress is a series of leg burning bumps.

Practice climbing lesser grades, say 4%-6%, seated, in much too high a gear, so that your cadence is very low, in the 45-55 range. Hold your upper body still and rotate your legs smoothly, pushing forward at the top, pulling back at the bottom, maybe even pulling up a little on the backstroke. When you get the hang of that, just sit and do that in your lowest gear on that steep climb. I don't usually stand in any ring under 39T. It's easier and more efficient to sit in very low gears. We have 26 X 34 on our tandem and have climbed 19% grades, fully loaded, with that technique.