UP2718Q, calibration while in HDR modes?

Unknown. The Displays Team recommendation is to calibrate using DUCCS in non-HDR modes. I have not seen any documentation on calibrating while in HDR modes.

By definition a Rec2020 HDR calibration for a display should be a LUT3D calibration that include clipping policy for out of gamut values (colors in input that your monitor cannot show).

DUCCS works with LUT-matrix-LUT structure which IS NOT a LUT3D. A LUT-matrix-LUT structure is meant for limiting native gamut to sRGB or AdobeRGB or just keep native gamut and fix whitepoint and gamma value.

"HDR" modes from several manufacturers are a factory non writable LUT3D-like calibration that does the mapping described in 1st paragraph. So... you can't update it.

It is possible to build a software LUT3D that will work as a calibration for a few selected software solutions like Davinci Resolve or madVR video render. Two flavors:

- Rec2020/HDR "in" -> your monitor SDR capabilities at native gamut "out"
Just get your monitor the closest you can to desired white (usually D65), keep native gamma and maximize contrast. Usually you can get it in Custom color mode since DUCCS calibrations in CAL1/CAL2 are not maximum contrast friendly.
Once you have done it, just profile your monitor without calibrating it with DisplayCAL tool (free multiplatform software, google it). It means to just capture in an ICM file how monitor render colors.
With that profile you can build a software LUT3D HDR->SDR (keeping high % P3 coverage, not just Rec709 colorspace) to use with those programs.

- Rec2020/HDR "in" -> your monitor HDR capabilities at native gamut "out"
Same as above but monitor should be able to activate HDR input with Rec2020 mapping to monitor's gamut DEACTIVATED.
AFAIK Dells are not able to to that, HDR input and factory calibration cannot be deasociated. It's NOT recomended to profile HDR modes in an ICM file and the use it as input to generate a LUT3D.
Maybe I'm wrong, so ask Dell if it is possible to activate FALD in Custom Color mode and at the same time make Custom color mode acept HDR signal "In" without changing to factory HDR mode. If it is possible you can do a LUT3D keeping HDR contrast.
If not possible, your dell you have 2 options:

a ) trust factory calibration for HDR mode

B ) use a software LUT3D to render HDR content in a SDR screen (yours) with limited 1000:1 contrast, so for typical 350cd/m2 white you'll get  0.35cd/m2 back which is not good.

Anyway, your UP model's HDR and several other monitor manufacturer models' HDR is based in FALD (local dimming) with very few zones and 1000:1 static contrast ratio IPS panels ...which is a very dirty trick that gives you halo actifacts.
This is not an "usable" HDR by any means.. so even if your factory HDR calibration was near perfect (given contrast limitations) it would be useless and same goes to Dell competitors like Benq, Lg, Asus... those HDR models are useless as HDR monitors. You'll need OLED or at least very high contrast VA panel with thousands of local dimming zones.
Choose the lesser evil.