Rick-

In a word, no they can't. There is no drop-in octane booster additive that does everything TEL (tetra-ethyl-lead) in 100LL does. In addition, the piston aircraft engine industry does not have the kind of money and resources to launch an attempt to develop and certify drop-in cylinder head replacements for the existing piston engine fleet. This was possible for the auto-industry in the 1970s. The business case the automobile industry enjoys for this is a very different scenario.

1) The TEL (tetra-ethyl-lead) additive in 100LL Avgas is put there to boost the octane of the fuel. Airplane piston engines need a higher octane rating fuel because they run high-compression engines. High compression engines operate at a higher percentage of their total operating speed range. They need the higher octane and compression to permit higher mass flow rates used with supercharged and turbocharged engines, and to avoid the potential damage from detonation. Cars do not have these features although some of them are used in racing cars. In a word, aviation engines are high performance engines and they need high compression fuel.

2) High octane fuel gives less detonation. Less detonation gives better power extraction from the fuel, better burn performance, less wasted energy, and less damage and stress to the engine. Aircraft engines are safety critical engines. They need the extra reliability afforded by high compression, low detonation engines.

3) You can't run high octane engines on low octane fuels or damage will result. Some low-compression aircraft engines can, with a host of caveats and considerations, obtain an STC to operate with auto gasoline or mogas. As we know, the airplanes designed for use with 100 and 100LL tend to have very long service lives. They do not phase themselves out the way cars phase themselves out over a decade or so. You need to have an octane booster to replace TEL (100LL) if you want these engines to survive for multiple decades. To date, no such replacement has been invented. A large percentage of the existing fleet can use 91/96 unleaded gasoline, perhaps even 70%, but there is a large number of airplanes not served by this substitute fuel. Those engines are true high performance engines that need the high performance characteristics associated with the TEL additive.

4) For decades TEL was used to boost the octane of fuels used in cars and trucks across the modern world and allowed the production of very strong engines. High compression engines made before the ban on leaded automotive fuels are still prized by performance car enthusiasts for the extra horsepower they put out. In 1973 the toxic effects of TEL were shown to be the case, and as a result of this and the fact that lead interferes with the action of catalytic converters used for emission reduction the EPA won regulation against the use of leaded automotive fuels. That's a good thing supposedly because lead in the atmosphere is a very harmful, a topic for other discussion. Engines went to lower compression ratios at an expense in the millions in terms of research and redesign for engines for cars. Avgas remained exempt from the law because developing new cylinder heads for small aircraft engines would have cost more than any of the major engine manufacturers could bear. There are high design, manufacturing, and certification costs associated with aircraft engines and they tend to change very slowly in comparison with automobile engines. It is a smaller industry and it cannot afford to accomodate the rapid changes imposed on the auto industry. At this time, a drop-in replacement for 100LL fuel is what is really needed.

5) TEL in 100LL also cools valve seats and prevents corrosion in the engine. Carmakers had to come up with tougher valves and valve seats that could withstand the higher temps of detonating fuels and the corrosive effects of unburned mixtures. This could be done for aviation engines as you suggest, but it's really not the main issue. High compression ratio-compatible fuel is what is needed at this point.

6) There is a lot of hope in this regard. Funding for alternatives to 100LL is ongoing and is fairly well funded. Some notable programs are the Swift Fuel invention from Purdue University, and the FAA research on the subject being conducted at their New Jersey facility.

Here's a little bit of sleep-inducing material on the subject.

AOPA on leaded gas
AOPA on 100LL
Wiki on aircraft engines
Wiki on avgas
Wiki on gasoline
Wiki on octane