True, old fashioned flash (Unomat, for example) has about 200V at sync connector due to circuit design. To prevent 5D from electrical damage there are many simple DIY solution. The most expensive elements are optocoupler and triac - less that $10 even if you buy them separately on eBay. The circuit consumes minimum power and can be powered from separate battery, adapter. You can integrate it into your flash bulb device. I will be glad to share my experience in photography and electronics.
Best regards,
Ivan