A possible solution maybe but given the OP lives and works in the UK seems a bit extreme to appear to give that up to get an IEC permit. The OPs resume will reflect they are living and working in the UK so the last entry will need to reflect moving to Belgium with or without a job plus they will be declaring on the application that their country of residence is Belgium. As for the passport submission that is not an issue for the IEC program for visa exempts given the whole process is managed online through a MYCIC account.
Ultimately I guess is up to the OP how they might want to stretch the rules but is so easy in these situations to drift into a misrepresentation scenario, maybe not with the initial application but later on with any future dealings with immigration so any action take now needs to be consistent throughout in the future.
Ok so this situation would be similar to a situation when somebody asks for inland spousal sponsorship days after they have entered Canada as a visitor.
Since the only requirement for such sponsorship (inland type) is to be in Canada, bur not specified how long, they can do it just a day or 2 after entering.
Now the requirement for the working holiday is that at the time of sending application that person has to reside in the country.
Which in case of Belgium would translate info following:
1. To be there physically at the day of the application was sent (that can be done very easily given that UK and Belgium are neighbor countries
2. To have a residency permit. Since the other condition is to be a citizen of Belgium, so this permit would be a permanent residency permit. So as long as that person still has a permanent residency address there, there should not be a problem.
Now what is requested are working holiday - a program that takes much faster to approve and one has to use it rather fast after that. (we are speaking about 3-6 months from the time of application until that person will go to Canada).
Of course once you send your application (and all is OK there), you do not expect to stay in UK anymore, so transition through Belgium should not be problematic (to stay there during the application time).
Situation would be different for somebody who does not have permanent residency in Belgium (a rules to get it might include long term rental contract for example).
