"If we use an engine oil with a high base number, can a commercial engine cleaner help remove the sulfated ash from the valve seat?"
The first recommendation would be to consider using an oil with a lower base number. New engine oils usually have a base number range of 5 to 15. Most modern diesel engines (manufactured after 2007) that comply with the ever-increasing U.S. Environmental Protection Agency (EPA) regulations use oils with a reduced base number. Some may be as low as 0.1 percent per volume. This is so as not to clog the diesel particulate filter (DPF). Sulfated ash does not burn off and can block the fine pores of these types of filters, causing exhaust backpressure and a loss of power and fuel economy.
An engine oil's sulfated ash content is also directly related to the oil's acid-neutralizing capabilities, as the base number for most engine oils comes from the metal-containing detergent additives. Generally, the higher the oil's base number, the higher its ash content and the greater its ability to prevent acid corrosion in the engine, which is a good thing. However, the mandatory use of ultra-low-sulfur diesel fuel on highways has reduced the need for higher base number additive content. This is why you should consider switching to an engine oil with a lower base number.
As far as commercial engine cleaners are concerned, there is a wide variety of these types of products on the market from a number of different companies, many of which make unrealistic claims about their ability to help engines perform like new. Of course, there likely are a few good engine cleaners out there, but it can be challenging to identify which ones are effective.
One final suggestion would be to try a good synthetic oil to clean up the ash deposits. Synthetic oils can clean up many of the deposits left behind and can keep the engine running clean. In fact, the performance of synthetic oils is supported by a considerable amount of evidence. The only problem would be if you have an engine with high mileage and a lot of deposits. In this case, the engine may start to use more oil as the ash deposits are being removed. The oil could then move past these worn parts, i.e., cylinder walls and valve guides that have been previously blocked by deposits.