You are on the right track if ASME Section IX is the applicable document. Test position is a nonessential variable when qualifying the WPS. Likewise, product form, i.e., plate versus pipe, is a nonessential variable. You can qualify the WPS by welding a test assembly consisting of either plate or pipe. You can qualify the WPS by welding the test assembly in any position. The WPS is qualified for plate or pipe and it is qualified for all positions. The exception is when notch toughness is required by the construction code, then as you noted, the supplementary essential variables come into play. Usually the WPS is qualified using the maximum heat input. That is accomplished by welding the test assembly in the vertical position using uphill progression.

My only advice is don't get to creative when reading and applying the provisions of the code. Play it conservation. That an extra step or two to be safe. There is nothing more damaging to your reputation or your employer's reputation than to discover the WPS's are not qualified properly and the product is rejected by your customer.

The ability to properly qualify and document the qualification of the WPS is a measure of your technical expertise.
a little extra time making sure the documentation is in order pays dividends.

I will also advise you to backup your PQR by attaching copies of the CMTR for the base metals used and all laboratory reports. They should become a permanent part of the PQR. It lend credence to you documentation even if it isn't required by the code.

Best regards - Al