Hi,
depending on the configuration of the transmitter <-> antenna construction, there are at least two hazards (as written above, just for clarification once more...)
1. Transmitted RF has a resonance frequency on a resonance wavelength.
(And some multipliers above and beyond that, their strength depends on the quality of the used gear, mainly...)
This transmitted RF is usually much higher when HF is used than in VHF/UHF applications. This has to do with the benefits of higher wattage in HF (the gain in the higher frequencies of VHF/UHF is not as large as in HF's frequency regions).
The transmitted RF does include very high voltages and very high currents. (Almost) never at the same place but ... see on (2) below :-)
RF will bring everything in (electrical and/or magnetic) resonance that fits its needs. If an HF radio transmits - for example - in the 20m wavelength region, there'll be resonance on every metallic object that is 5m long or a multiplier of this (this is a cruelly simplified explanation). There can be even resonance on shorter or longer objects due to special effects like electrical or magnetic shortening or increase in e. or m. length due to capacities etc.
The argument with the Faraday Cage is not valid when the RF is emitted by the refuelled object. It's thinkable that an HF transmitter sends on a fixed frequency and the wing which has a fixed resonance frequency is 'adjusted' by the fuel level (think of a fuel tank as a large capacitor with a very large distance between platters and then fill the capacitor with Jet A1... ;-) )
There are countless possibilities of resonating airframe, malfunctioning fuel pumps, high induction currents on the pumps' motors etc. - all created by a simple coincidence in resonance between your HF radio's frequency and ANY object on the airplane that resonates on this frequency.
2. There's no ideal antenna.
First, antennas are always imperfect. There's no practical antenna solution that transmits all of the HF energy as resonating HF. There's always some electricity 'left' due to a wandering standing wave. (SWR Standing Wave Ratio)
This electricity usually works as an antenna heater ;-)
If there's the slightest problem with the antenna, this current can become really significant (almost every HF problem is due to antenna related problems, aerial RF that is...) - imagine this current finding a way from the HF antenna to your fuel, finely vaporized whilst refilling your tanks with high pressure... all you need is ONE broken insulation or a 'high quality Seattle soldering' out of place... ;-)
3. Jet A1 is not Halon.
One has to differentiate between a puddle of Kerosene on the ground and finely vaporized K. in a fuel tank. The latter is by far more prone to a spontaneous adventure in exothermous reaction... ;-)
What I want to say:
Please don't be mindless. There may be only a slight possibility of an incident regarding HF with Kerosene or other fuels... BUT... it's really possible.
Personally, I would not jeopardize my career, my plane or the lives of the people involved for a simple 'I don't believe in that'.
Kind regards,
Peter
PS: Even if the fuel has no problem with your el cheapo HF with its crappy antenna on your multi million dollar airplane. Seeing your fuel specialist falling down his ladder with severe HF burns on his fingers won't make your day, believe me. These burns are nasty and painful as hell and they tend to take a long time to heal, similar to fluoric acid. Don't play at work!