With a bit of experimentation and practice, you can probably do this.
First of all, my oven has a mode where it only has the light and fan on. That's useful for proofing, because both consume energy, thus generating heat. They're sufficient to maintain the oven around 30-35 degrees, but they would take forever to heat it to that temperature.
So when I need to proof something, I turn the oven on for precisely 45 seconds, then turn it to the "light and fan only" setting. I've found that if I'm starting from a cold oven that's enough to bring it to approximately the right temperature (not right away -- the heat needs to diffuse from the element). An infrared thermometer can help you find the right time, as can simply feeling the walls of the oven after ten minutes diffusing. Even if your oven doesn't have a setting like that, an initial blast of heat can maintain a warm temperature for a while.