Well, let us envisage an implementation using our current computing technology. A computerized system using the internet from a technical point of view would not be difficult to develop, as many of its parts and problems have already been solved by other applications and platforms. A web interface with a personal account that can be accessed like an investment portfolio or a bank account is what one would expect to see. Let us say that a person's tax money (at least a certain allowed percentage subject to government permissions) can be directly assigned to various projects by the account holder, as one would do by picking company stocks on the open market. These projects are proposed and backed by government officials, and they are set to compete with one another as private companies do. For the tax holder, he can allocate his money to the projects that he believes the tax money should go to. If in the instance where the tax holder does not care and does not allocate his funds, then a suitable default allocation is made (perhaps based on a set of preferences to auto-allocate). Such a system gives direct transparency to the tax payer on where his money is going. If he is more inclined to support education than infrastructure then he can make that allocation. Additionally, if the money trail itself can be tracked like a package (perhaps blockchain is applicable here), then it should provide even more transparency to the taxpayer and decrease the likelihood of money laundering and shady reallocations further downstream.
The framework would also allow for campaigns by public and private groups to support certain projects and to convince people to change their allocations. I.e. advertisements to win your tax money. This can be used to foster competition within government contracts and improve quality. It can also let the taxpayers interact and bring focus to important issues such as local infrastructure needs and EPA violations. Therefore it has the fluidity and the functional means of allocating funding for things that directly matter to the tax payers, rather than allowing representatives to interpret and spend that money as they deem fit. However it's important to note that the implementation of such a system and the degree of freedom allowed for each tax payer will be dependent on the maturity and the culture of the society, in much the same way how governments differ for different societies, even though they may all like to refer to themselves as "democratic". But it does have the potential of decentralizing the echelons of power by taking direct control of government funding and projects. Therefore in its most progressive form, it would essentially be a direct democracy.
Conversely, such a system in all likelihood would be incompatible with existing power structures as it would create more problems than solve them. Especially when you consider that all nations run a deficit, hence uses a portion of the tax money to pay down the national dept, or that taxes are used to fund projects that may not be popular with the general public but are deemed necessary to the insiders. Letting the average tax payer make these decisions for themselves might incur national security problems. Rather my personal take is that this is suitable for societies that value transparency in governance, composed of a well educated and emotionally mature population who value individual freedom and social cohesion.
assert(q); // TODO: Verify assumption