It`s a bugger when that happens,, the pilot screw/mixture screw is a solid screw and slightly recessed and no they won't be poking out inside the carb so no using your mole grips. I don't understand how it could seize as it ought not be tight, it has a taper on the end to meter fuel and a spring between it and the carb body to put tension on it to prevent it vibrating in and out when the engine is running so putting the adjustment out,, it should be in the region of two and a half turns out from lightly seated.
I`ve used this method of removing stuck/seized solid carb screws in the past,, with a steady hand drill a very small hole down the centre of the screw ( its brass so should drill easily) 5mm or so and then use the Tang end of a small file ( I use fine jewellers or modelling files) and lightly tap it into the hole you just drilled ( the hole being round and the file Tang being square or triangular ) it should grip the screw,, you then use your mole grip or pliers to grip the end of the file and keeping pressure on it short sharp anti-clockwise motion to shock the screw and hopefully it will free off,,, heating the carb body by immersing in hot water for a time would also help,,, take care with the file though mate as they are very hard yet brittle and can snap easily.
I use these for carb jets,, http://www.motocarb.com/
But you can get them many places including E-bay
I recommend you use genuine Mikuni jets for this reason,, Mikuni jets and after market jets tend to be marked and categorised differently e.g, a Mikuni N100.604# 105 Is marked according to its flow of fuel ( its flow rate) that is 105=105 cc of fuel per minute ware as for example DynoJet are marked according to the diameter of the hole through the middle of the jet so a DJ150 = 1.5 mm diameter. Another example is DynoJets have flat machined surfaces ware as Mikuni jets are chamfered on the inside opening of the threaded end so there are physical differences as well ( I have a chart some ware with cross references) CARB SCREWS THREADS
8 x 4mm both float bowl
4 x 5mm top cover
4 x 4mm top cover
2 x 3mm needle plate
2 x 3mm needle plate
Measure the length and you can get them anyware and yes even E-bay, I recommend stainless steel cap head then you can use an allen key.
These are the sizes for a marauders carb screws so I guessing they are the same.
I also have made an assumption that the vl800 has a similar pilot screw setup to the VZ800.
Hope this is of some help mate and good luck.