I mean with respect to the effects of having large (in comparison) quantities of the drug dissolved in the blood. Activated charcoal is very good in cases of poisoning but it only removes toxins from the GI tract, not the blood. In the case of an overdose, the person wouldn't have enough time to wait for the drug to be excreted via the urine but in cases where the person needs to get rid of a the drug with a long half life, urinary excretion is the only route I know of.
As for urine drug testing, I believe that it depends on how lipophilic the drug is. Lipophilic drugs like THC deposit in fatty tissue and consequently can be released weeks and even months after ingestion. Same thing applies for blood testing. Similarly, some drugs deposit better in hair follicles than others. Drugs get deposited in the hair as its being formed so a strand of hair from a long haired hippy would contain quite a history of drug abuse lol.
Urine formation is the result of glomerular filtration and then some fluid is reabsorbed, the rest goes on to be peed out - (a diuretic prevents absorption). So it really depends upon how the drug is handled by the kidney. If the drug is freely filtered and neither absorbed nor secreted by the nephron then a diuretic would have little effect. Reason being that the elimination is linked to glomerular filtration rate rather than urine volume.
If the drug is freely filtered and also secreted by the nephron - then changing urine volume with a diuretic would have little effect. Again because elimination would be more related to GFR and secretory rate - which are unaffected by a diuretic.
If the drug however was freely filitered and then absoprbed by the nephron - then indeed a diuretic would speed filtrate transit time and may reduce drug absorption and may increase the elimination rate of drug. Can I think of any examples off the top of my head? No.
There are several factors that must be considered.
As stated before, each drug has different windows of detection.
It also depends on what model of drug testing is being considered.
For example, in regulated testing (such as DOT testing) there are specific thresholds set by government regulation which play an important role in "windows of detection".
The biggest impact that a diuretic will have is in "peri threshold" concentrations of drugs. For example, if the cut off for reporting is 300ng/dL, a diuretic could push that concentration down to 299ng/dL, which would be reported as negative. Scientifically, there is no difference between 299 and 301 except one gets called negative and the other is called positive.
Virtually all of the products available on the net that are reported to enhance excretion actually work by volume loading (typically there is a paragraph somewhere that recommends taking the product with "a minimum of 1 or 2 litres of water" which is a clever way of saying "volume load" to dilute the sampel).
Final comment relative to diuretics. Virtually all tests assess specific gravity and/or urinary creatinine as indirect measures of hydration. Many laboratory tests include tests for diuretics since they are commonly used to cheat the UDT process.