Yes it is. Drinking nanoparticulate silver is also, pretty hardcore-- just in a different way.
https://www.ncbi.nlm.nih.gov/pubmed/27518167Now, I doubt that the kind of powder being mentioned here is in any way nanoparticulate in nature--- (if it was,
the resulting holy water would be a disgusting brown color) so that is probably not an issue.
I would however, point out that DnD's use of silver in this capacity is linked to magical practices and beliefs about silver being a purifying metal.
http://www.witchipedia.com/mineral:silverThus, the addition of the silver is more to purify the water, so that it can better be a vessel for the positive energy imbued into it by the ritual performed by the priest-- the immense amount of silver required to make a vial of holy water, being linked to the limited cleansing property of the silver itself-- you need a lot to really clean that water what good before it becomes suitable. That's my understanding anyway.
Much like really powerful wizards can gain the ability to perform spells without material components, I would say high power clerics should be able to perform similar feats, and create holy water without first cleansing the medium in this fashion, through the power of their immense faith, and direct connection with their deity. In such a case, a high power cleric declaring their URINE to be HOLY WATER should be considered legit, IMHO.
Edit:
I would probably require that they be able to perform miracles before being able to do something like that-- and that they are in fact using the create miracle ability to convert their urine into holy water, and that is how they are sidestepping the material requirement. In such a case, their urine would be genuine holy water, and would not be salty or yellow. It would be pure, perfect, holy water. On tap. I would charge them the 5k EXP, and the equivalent cost in EXP for the 5lb of silver. I would give them a boon though, and ask if they wanted it to become permanent, as a consequence of being a direct conduit of their divinity of devotion. Being able to piss a glass of holy water would be useful at times, but would also be dangerous, because they would always exhibit a positive energy aura, from containing a "potion" in their bladder.