Sounds like your clients want results without paying for it. You sound willing to do that, using spammer techniques, but it is not a good idea for a couple of reasons.
1. Hiding a paragraph or two of content can get the site lowered or banned by Google instead of increasing its rank. Your customers won't be happy with the cost of recovering from that. And, you will get all the blame.
2. Hiding the reality of what it takes to be ranked high in Google from your client is a dis-service to your client. They will expect more from you in the future (until they get pushed to the back of the line in search).
3. Not educating your client on the realities of needing good visible content their customers, not just the robots, can see, reflects poorly on you. Anyone in this business encounters difficult clients unwilling to pay for services is a given. It's a good learning curve for you, more than for them.
I have a client who has just come back to me and my recommendations after a year and a half of trying to get results from others promising them more for less, but never able to deliver.
When you have the confidence and experience to know what your knowledge and skills can do for your customers, and know that what you requires monetary value, than you can calmly lay it out for them.
One option is to let your client know what kind of ranking they can get without adding content to their webpages. Then let them know what ranking they can expect by adding content and at the price you require to do the work and stay in business.
Performing black hat/spammer techniques falls on you, not your client. It's too high of a risk and price to pay for a client looking to get ahead by squeezing you dry.
