i think your only option might have to be to wire it into the seat switch, this wouldn't get rid of the delay though. It would just require them to buckle up so the operator presence system would operate and enable the truck while they were on it and disable it when they leave the seat. It does this already with just the seat switch.

Is there any option to retro fit a seat belt with a buckle sensor that would also cut out the traction?

This of course requires the operator to use the seat belt correctly at all times & not just fasten it behind them.

not sure what fortis truck your working on but this may help some.
check the inching brake/transmission overlap settings, they set the time delays related to when the transmission engages/disengages.
it could be it is set to a setting that creates a slight delay in the transmission clutch pack release/engage reactions.

SET INCHING/BRAKE OVERLAP
This function is available on lift trucks equipped with a two speed Basic Powershift transmission shown below
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S30-35FT, S40FTS (E010, F010)
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H1.6-1.8FT, H2.0FTS (H30-35FT, H40FTS) (F001, G001)
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S2.0-3.5FT (S40-70FT, S55FTS) (F187, G187, H187)
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H2.0-3.5FT (H40-70FT) (L177, N177, P177)
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S4.0-5.5FT, S5.5FTS (S80-120FT, S80-100FTBCS, S120FTS, S120FTPRS) (G004, H004, J004)
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H4.0FT5/FT6, H4.5FTS5, H4.5FT6, H5.0-5.5FT (H80-120FT) (N005, P005, R005, S005, U005)
Use this function to change the amount of overlap between the brake and the transmission. The overlap can be adjusted using a scale of 1 to 10. Using a setting of 10 will result in the most overlap. Using a setting of 1 will result in the transmission being decoupled before brake system pressure begins to rise (no overlap).
Typically, overlap will be set low if high rpm hydraulic operation is required at low travel speeds. High overlap is typically used to hold the lift truck in position at low travel speed when operating on uneven or sloped terrain.

like brewski suggests there is no 'time delay' setting you can change, that delay that is programmed into the system is exactly what he said, it is set this way just in case the seat switch is opened in situations like he said, if the operator gets bumped up off the seat so it is programmed to allow for this scenario.
All you can do is change reaction characteristics, it may effect the timing somewhat, it's just something you will have to experiment with and set it to what works for you the best.
Also you should probably let a hyster tech do this for you if you are not one. You can set this through the dash but it would be alot easier using the pc.

I was told the delay allows for an operator to be able to continue operating the truck if he momentarily raises off the seat due to hitting a large bump or leaning enough to open the seat sensor. This stops the truck from coming to an instant stop if this occurs. There may be an allowed change in the delay. You will have to contact your local dealer and have them contact the manufacturer to see if they will write a updated CDF file for the truck. The time delay is not adjustable in the truck setup.