If you squawk 0030 (which means FIR Lost) D&D will know immediately what your problem is (unlike 7700) and where you are before you even speak to them on 121.5. Once they've "got hold of you" you'll be instructed to select 0031 (FIR Found) or a discrete D&D squawk.
0030 is in the SSR Code Allocation Plan.
I've spoken to several D&D controllers about this. Please DO NOT squawk 0030 unless you are instructed to. It is only for use by the FIR sector and the AIP does not specify you can select it without an instruction to do so. If you select it, then it will indeed flash up on D&D, however they will then immediately be contacting the FIR sector to find out about your aircraft. If you are inside CAS, then the ATC unit whose airspace you are in will also be calling the FIR sector, since that is who they think you will be in contact with. All your unauthorised selection of 0030 will do is waste time and effort at the various ATC units (including D&D) who will be trying hard to ensure your lost aircraft poses no threat to other traffic under their control and find out who you are.
If you do become lost, then D&D advise you to initially report your lost status with the ATC unit you are in contact with. They can then co-ordinate a plan of action with D&D to provide appropriate assistance. Squawk as instructed by ATC. If you are not in contact with anyone, then D&D are more than happy to take your call on 121.5. Further, if you are making your lost call initially on 121.5 AND you think you might be in CAS somewhere, they have absolutely no problem with you squawking 7700 immediately. This will not only alert them quickly about your possible aircraft position, it will also alert other ATC units who can then take action to safeguard their traffic before further co-ordinated action is taken by D&D. The important thing is for everyone to recognise that your aircraft is in difficulty (even if only navigational) and to ensure that everyone else is kept safely away from you. The exact nature of your problem will become apparent to affected ATC units as things develop, and as D&D carry out the job they are highly trained for.